Ok. I actually did the write-up on clubgp. Here is a copy and past of it.
I have both a 2002 Pontiac Bonneville SSEi and 2001 Grand Prix GTP.
Here is my comparison of two cars in stock form:
Style and Exterior Appearance
Both cars have sleek muscular body shapes and clean lines. The GTP is especially handsome in coupe form. I prefer the Bonneville to the GTP in sedan form. The Bonneville has an aggressive sculptured front fascia - and nose down/rear-up ready to pounce stance. The SSEi has larger brakes and wheels with lower profile tires - which enhances its appearance along with tighter panel gaps. The GTP offers a factory appearance package from SLP (GTX with ram air hood).
Cockpit and Interior
The Bonneville has a larger interior with richer materials and thick bolstered front seats. The GTP as a coupe accommodates a more performance oriented driving position and better visibility (longer side windows, no b pillar, larger side views).
Both cars offer a HUD and steering wheel radio controls.
The SSEi has a analog boost gauge which actually displays boost between -10PSI and 10PSI.
The GTP uses an "arch" of light bars in the driver information display that represent "minimum" to "max" boost. The SSEi has automatic two zone climate control, and rear seat vents. Both offer available heated driver seat and full power driver seats. The SSEi heated seat package is both front driver and passenger, and power is standard for both seats.
The GTP offers a factory interior upgrade package from SLP (GTX with carbon fiber dash and control accents).
The SSEi has an in dash tape deck and CD player. The two cars also offer MONSOON sound systems, and optional 12 disk trunk mounted changer. I have the MONSOON in the SSEi - it has amazing sound quality.
The GTP is lighter with same engine and transmission. However, the SSEi is available with a higher (3.05 vs. 2.93) final drive ratio. The GTP also use to be rated with the same torque peak but at a lower RPM (3200RPM vs. 3600RPM). The GTP has more weight over the front wheels which makes it easier to launch hard from a stop. The SSEI also does not have a ubend and appears to have a freer flowing exhaust.
A stock GTP will probably be a half second to a second faster 0-60MPH and in the 1/4 mile than a stock SSEi. An SSEi with a 3.4 supercharger pulley will be as fast if not a little faster than a stock GTP. I was driving just that combination for a few weeks. The SSEi is an overall better car with this pulley, than a stock GTP in my opinion. Hence my upgrades to the GTP
The majority of club members rin between 14.3 and 14.7 in the 1/4 with a stock GTP - you probably won't find anyone higher than 15.0 or lower than 14.0 with a stock car. Scott Cook ran a 15.8 stock with his 2000 SSEi. An owner on another board claimed he ran a 14.8 stock with an SSEi. The majority of auto mags get 15.0 for the GTP and 15.7 or 15.8 for SSEi.
The GTP offers a factory smaller pulley (3.5) and performance exhaust from SLP (GTX stage II).
The 2004 GTP will still be powered by 3800SC engine. However, it will supposedly deliver between 255-280HP through induction, exhaust, and supercharger improvements. I believe its basically port and polish stuff, the GMPP exhaust, and some PCM tweaks. The current info from GM says 260HP, but the show car (g-force) had 280HP and this is the unofficial target.
The 2004 Bonneville GXP will come with a 4.6L version of the 32V DOHC Northstar V8 -possibly with variable valve timing. The engine is reported to produce at least 285HP and similar torque. I believe the target is 300HP.
The cars both really excel from a roll between 20MPH and 120MPH
The SSEi has bigger brakes and wider rubber, and stops shorter, quicker, and straighter than the GTP even though its a heavier car. Both cars have full speed TCS and ABS. The cars both brake very good even with stock rubber, better tires will let you out brake or at least brake with nearly anything on the road. There is a larger variety of aftermarket upsized brakes for the GTP. You can order an SLP (GTX) or GM package (GMPP) for the GTP from the dealer with bigger front brakes (12" vs. 10.9"). I would not go by mag braking numbers with the GTP. They get 130-140 ft for 60-0MPH. I was able to routinely brake with my friend'* Volvo C70 (which has full aftermarket brakes). The mags get 118-128 ft for 60-0MPH with a stock C70.
Handling and Ride Quality
Both cars are extremely stable at all speeds and have predictable and easily controlled handling. The SSEi appears to have higher ultimate grip, better turn in, and better responsiveness even though it is heavier. The car has a more advanced chassis and suspension - shared with Cadillac STS but supposedly tuned for greater performance.
The SSEi also doesn't exhibit as much understeer at or near the limit. SSEi handling is further enhanced through Stabilitrak. Stabilitrak is GM'* dynamic stability control system is widely considered one of the best from any manufacturer. I can testify to its performance, it kicks in just when you need it and corrects understeer, oversteer, or drift without slowing you down. I am more willing to push SSEI to the limit due to the confidence it inspires - especially in an unfamiliar corner or in wet/ice conditions.
The Stablitrak system works by comparing your intended path (based on steering angle) with the lateral acceleration (cornering rate) and yaw rate (angle of nose). The system will then apply one or both brakes to correct for drift (nose straight with lots of lateral acceleration), understeer (steering angle big but nose angle small), or oversteer (nose angle big but steering angle small).
The 2004 GTP will offer an advanced version of Stabilitrak, as will probably the 2004 Bonneville GXP.
The GTP is more maneuverable if less agile due to its smaller size. The GTP also offers factory suspension packages from SLP (GTX with sway bars/springs) and GM Performance Parts (Koni struts/springs, sway bars, strut tower braces).
The 2004 GTP and Bonneville GXP will probably also offer upgraded suspensions - most likely Koni struts, lowering springs, and thicker stabilizer bars. I believe the 2004 GTP with "competition package" (COMPG) will in fact use the new GMPP Koni struts and springs.
The SSEi has a much more comfortable ride, and isolates you from road irregularities. The GTP is smooth and stable on highway even at triple digit speeds, but beats you up pretty bad over portholes or broken pavement.
The GTP and SSEI have a common drive train. Hence almost all engine or transmission aftermarket parts will work with either car. The SSEi does require some more work to install some things due to shape and size difference of engine bay - specifically CAI or headers. The biggest disadvantage is there is not currently a DHP chip for the SSEI. You can have a custom chip made for it but its relatively expensive, and you need to drive out to their headquarters. A DHP chip not only adds power and improves shifting but makes it easier to use higher final drive ratios and take full advantage of mods that increase power in the 5800-7000RPM range - such as cams, rockers, pushrods. These mods also improve power in the 4500-6000RPM range which you will be able to take advantage of with stock SSEi chip or GTP. SLP has a new hypertech programmer for GTP - which may work in the SSEI given its the same engine/computer just with different "code".
The GTP has a far larger aftermarket for suspension, brakes, and exhaust. However, the SSEi has arguably better stock suspension, brakes, and exhaust. The release of the GXP in 2004 may result in a GM Performance Parts suspension, exhaust, and brake package for all Bonneville'* based on the current body style.