If it's only 15 bucks then I would say just go for it.
If you are going to use the GTPSP as a way to fill in the GT when you're out of house then it does an ok job. I wouldn't say that it has lasting value like GT3 though.
I do not agree. Although you can see GTpsp has pretty low resolution textures in the environment, the cars look rather good. And the physics are not matched.
I want to take back my earlier comment about gtpsp looking rubbish when played on a ps vita, as it turns out I think I hold my vita a little too close which can make it look a little pixelated but with the console held at full arms length it looks really good.