I'll say FM4 is better than GT5 in all aspects, especially as a game. On this last regard GT5 had a million major annoyances and the game/fun factor is the main one for me, which is why I sympathize with CorvetteConquer.

There are three threads in this very forum, with subjective and objective approaches on the matter:
http://www.gtplanet.net/forum/threads/fm-vs-gt-discussion-thread-read-the-first-post-before-you-post.209534/
http://www.gtplanet.net/forum/threads/forza-4-vs-gt5-read-the-first-post-before-you-contribute.276308/http://www.gtplanet.net/forum/threads/forza-4-vs-gt5-physics-read-the-first-post-before-contributing.226995/

At the same time FM4 had a considerably higher metacritic score (reviews).
http://www.metacritic.com/game/xbox-360/forza-motorsport-4
http://www.metacritic.com/game/playstation-3/gran-turismo-5