If it were me, I'd stick with the SSEi. You've bearly got the thing broken in and it'* paid off. Now it'* time to start modding it up. For equal money in mods, the L67 can completely kill any N* out there (save the new STS-V with the blown N*). That and basically, the GXP is an SSEi with a N*. I don't believe that much, if anything is really all that different in the suspension and the interior (save the embroidery-sp? & guages) are practically identical.
Also, I am a bit biased against the N*. My family has had nothing but cooling system trouble with the 3 very well maintained N* cars (2 STS'* & a Deville) that they've had. I'll stay away from them like they've got the plague.
In the end, you do what suits you best because it'* your choice. Be sure to inspect the paint VERY closely as we've had quite a few GXP owners that have been unhappy with the paint coverage.
Trade in value for your car (based on Kelly Blue Book
& excellent condition) would be around $12,000 and private party sale should be around $15,000.