The driving aspect is arguable for both platforms. I'm really tired of the physics and driving models debates. We know alot about this stuff by experimentation, while both physics engines are not perfect, they are good enough on the console sim level.
As for the playability of Forza Motorsport, you will find that menu reactions and the logistics are done cleaner and faster than in GT (I'm the type of person who likes a good UI, but can bear with something a little bit sloppy a la GT5, although contrary to popular opinion, I like the GT home screen look). Where F4 really shines is the content and the value of the game itself.
Lets start with the offline portion. We have a whole variety of cars to choose from (although I wish they had GT's Kei-cars

) ranging from the lamest subcompact cars to supercars all the way to the most exotic Barrett-Jackson Auction Pieces. Simply, there will be a car for you in Forza. (If you were to get Forza, I personally recommend the DLC via Season pass. It has just given so much value per "unit" entertainment) T10 does a great job of taking interesting cars and implementing them as DLC (Porsche Pack, May 22!

). The paint booth with the livery editor gives endless possibilities; and I think you know all too much about this 'temptation'.
Now to online; Rivals mode is where Forza shines its best. Quite akin to NFS' autolog. It provides a really addictive game mode where it is so compelling to attempt to beat you rival. It is really hard to explain but there is some competitive high that is activated during this mode. The storefront and auction house are very convenient tools to obtaining decals, tunes and a wide range of vehicles (The AH isn't all rosy, I stopped using it because of the dirty credit issue). Online is not all rosy, as of late I haven't been able to hitch onto a decent lobby and maintain through a race (with the exception of a custom lobby). I'm disappointed that I'm spending big bucks on XBL and getting dropped finding a lobby, or finding rooms that are completely devoid of life. And before the guns go out,
at least I get get a hold of active rooms in GT5. I dearly hope T10 will fix this up, but because online racing is not my priority, it isn't a deal breaker for me.
Content creation is really neat in Forza; although the inherent I/O system from the Xbox 360 causes the user experience to be much worse that it should be. I haven't used the video editor much, but looking at videos on youtube, enough effort and dedication can lead to spectacular publications. Photomode is spectacular in Forza; although I would say that the camera aspect handles better in GT5, the ability to pause the game at any moment to take a picture is a very nice feature that I wish GT5 had. Although shots have to be loaded into a XBL account which requires a Gold Subscription, and these shots have a hint of compression. As a result normal pictures from Forza in general will not look as good as GT5's, provided its of a Premium subject. Bigshots are very nice because they have no compression but they are a pain as my upload speeds from my ISP are terrible. The livery editor can be considered content creation, I don't think I need to elaborate here; If you are willing to spend the time and effort the results
will be amazing.
Regarding the Microsoft Ecosystem and the Xbox 360, the console handling is excellent. I bought my PS3 initially because it had Blu-Ray, Gran Turismo and an Interface I like. The original Xbox 360 Dashboard looked and felt awful in my opinion. Now inline with Metro UI, the Xbox handles very intuitively and is pleasing to look at and use. While I like XMB in the PS3, I feel its due for a refresh. Both devices are very capable media devices and it would be totally up to you if you want to sell the PS3. If you are not going to use it for its Blu-Ray or play any games, it might be time to trade away.
Sorry for being so long, and keep in mind alot of this is my own opinion. Hope this helps you. Just for reference I own:
- 120GB PS3 Slim (Generation 1 Slim) + GT5
- 4GB Xbox 360 (Generation 1 Slim + Matte Surface) + 20GB HDD + FM3/FM4:CE