Yes, but Traverse Town wasn't in Birth by Sleep. Did they recreate Traverse Town in BbS just for the trailer? Hmm
Yes, the 3DS is more powerful then the PSP. Expect the 3DS to have many more animations, more detailed areas and characters, short loadings, and more. With Resident Evil recently released, you can see that you can pull off 360 looking graphics with the 3DS with a few tricks.
The model for Sora is probably going to be the same one from the PS2. Although, I'm expecting them to polish up the model so it looks more tuned up.