Only ever played Ferrari Challenge on the DS, so I can't really talk about that in terms of physics. I'm in the SC beta, and I'm assuming since it appears to be open now that I can actually talk about it.
I think it feels better to drive than GT5P, but I put that down to GT being set up more for wheels than joypads. The cars all behave in the way you expect, although I don't think there's quite enough kick when you floor the throttle even with the driving assists off (probably to fit the console crowd better).
Are the cars in the beta all FR layout? I don't think I saw MR and certainly no RR cars on there, so it's hard to say if there's much difference in terms of handling. They're believable without being too realistic, if that makes sense. Unlike games like Grid and Dirt, it actually feels like what I do with the pad relates to how the car behaves, but it's not so unforgivingly realistic that you need to practice for hours to get your laptimes down.
I'd like to support System 3 as a small developer, but the list of options than Forza and GT have interest me more. I still prefer lower-powered cars anyway