As far as a game is concerned, I just think GT4 is better simply because it had plenty of things for the player to do.
When I start GT5, I just can't find anything to do a lot of the time. I honestly feel like I spend more time buying cars and going through menus than actually playing the game. Sure, it has online, but I rarely bother with it since I often have a hard time getting a race I want. Why GT5P's online system was scrapped, I'll never know. I would enjoy GT5 more massively if it's single player was improved or the matchmaking system in Prolouge was brought back.