it is hard to compare these 2 games in the sense that everyone is jumping to "ooh mine is bigger than yours, argument!".
GT5P is a race-sim to the fullest extent of the idea. On the other hand GRiD is all out arcade. However if you look at the racing experience, content, replay value, online play, and the full value of the package i'd would strongly recommend to go with GRiD. Check out my review of both of the games if you need extra info GT5p Review, and GRiD review
Hope that helps man!