The only way to make your decision easier for you is 2 test drive them both, i could say as a personel point of view 2 go for the a500 thats because i own 1 and know all about the car its easy 2 be biast, i have never drove a gpa although i think they are a wicked looking car i could not tell you how it handles, drives etc. If your willing 2 spend the money it costs 2 own 1 of these cars you need 2 be 100% sure 2 late once you`ve purchased. You would not not walk into a shoe shop and buy a pair of shoes without trying them on 1`st . Same rule applies . 

I drove both before I decided, and in the end it just came down to personal taste more than anything. Both have similar performance, but the A500 seems more 'cheeky' where the AGP is more 'racey'. And like others have said, you won't be dissappointed either way. I still smile every time I see mine parked up