Depends on what gen Aurora you want. Some prefer the Classic whereas others prefer the more conservative but equally capable 2nd gen.
The 4.0 comes standard with the electronically controlled 4 speed 4T80E, the best GM transmission offered at the time. You will find minor tweaks on the 2nd gen 4.0, its quite a bit more revvy and engine compression has been reduced to allow for regular unleaded fuel. The Classic 4.0 features 11.3:1 compression ratios and 91+ is recommened.
For interior fit and finish, its heads and tails above the Bonneville. The Aurora was designed for North America, ie, the driver oriented center dash panel will only work for left hand drive. Every instrument is clearly accessible without stretching your arms. Perfect, as I am 5'-8".
The Bonneville and Aurora are two very different cars. If you want balls out engine grunt and an aftermarket, then go Bonneville. You'll live with the interior as the power makes up for it. However, for a top end driving experience, V8 smoothness that gives 30mpg hwy economy, a proper tranny that spins up to 7000+ RPM without argument you'll go Aurora.
I tend to want to stand out in a crowd
Good luck with your choice!
Oh and if you need more Aurora feedback, www.auroraclubna.com