Yes, it should have better graphics and, yes, it will - since we'll see it on a better, more powerful platform, the new PlayStation 4. It's a given - it's almost impossible for a new game on PS4 to look the same as PS3, at the very least, as far as resolution goes. The problem with GT's graphics lies within its models, though, and the stance of its creators to driving, not from technical limits.
Firstly, they've got hundreds of models that haven't been truly updated since GT3. They just add a bit more detail and re-use them. Secondly, GT never gave a priority to the feeling of speed, but to its realism. Remember the first time we saw the "boost" shader in NFS underground, whenever we hit the Nitro? Yeah, if it was GT we'd never seen that. But we'd seen headlights with better halos around them, or stuff like that.
And yet, sometimes, some car models in GT6 are actually better than other "games with cars" in general. More detailed, more "correct", but maybe "not displayed as we'd like them to be". How's that? In smoke, burning, with other cars crashing and exploding around them. That's not GT, it never was, and I guess it will never be.
Forza, on the other hand, managed to offer a better mix of sim and "arcade-y" action, and for that it seems a better game to pick up. You'll play Forza for, say, one month and never feel bored, and it will be easier to "get into" as well. GT, any GT, is harder to "get into", feels "less shiny" at first and you may hate your first week trying to get to grips with its logic. But you end up playing it not for one nor two months, but for years to come. It feels "better in the long run", although, yeah, "more shiny" wouldn't hurt