I personally have found GT5 to have lost most of its flair and has generally become stale for me also. I don't play online much anymore as it usually is just a disaster. I normally just stick to doing the Seasonal Events and TT's and also take part in the WRS TT's. I still practice daily for about 5 hours for the next Academy. Other than that, GT5 has baisically run its course for me.
Forza on the other hand, I am finding myself putting more and more time into each day due to the numerous number of things there is to do in that game. I am neither a Forza or GT fanboy so when I say this, it is completely an unbiased opinion, but aside from the physics, GT just feels generic to me compared to Forza. They are both great games, but I think that if Polyphony doesn't come through with something good for GT6, I really think that Forza is going to be the game to have between the two of them. Its like GT has been stuck in the same old rut, release after release. They really need to revamp their whole gameplan and come up with some new and exciting things. Now I'm not saying they need to copy Turn 10's ideas, there are alot of other things they could come up with. And lets be honest, even with the physics being as good as it is, it is not without its flaws. Its like where Forza shines, GT fails, and vice versa.
Sorry if I've kinda went in another direction with my post in regards to the OP's original intent of the thread, just thought I'd give my two cents since I have and play both games.