I currently have both AT&T and Verizon iPhone 4 models for comparison, since I was already using both carriers. The other thing you need to know is I live in a huge major metropolitan area with great coverage from both carriers, so it's an ideal testing ground.
My objective is to keep the iPhone 4 that works the best, and sell the other. I'm carrier neutral so frankly I don't care if it's an AT&T or Verizon iPhone I just want the one that works the best. Especially when it comes to calls.
1) The AT&T delivers much faster data but we already knew that.
2) While slower, the data delivery to the Verizon phone is acceptable.
3) No
4) Call quality and clarity is just a bit better on AT&T, but again I expected this as my work BlackBerry on Verizon is a bit muted as is the new CDMA iPhone.
Again not a really big deal, just not as good as AT&T.
I rate these two as a tie.
They are both as good as it gets for the iPhone 4. I am totally convinced that if Apple will put the antenna back inside the next model like other cell phones do, then we will have the very best smartphone of all.
Right now my Androids on AT&T (Captivate) and Droid X (Verizon) make much clearer calls than either iPhone. That's not to be critical of the iPhone it is simply the truth.
Cheers...