I was at my sisters house last night and we were doing a test of my AT&T 4s vs my brother in laws Verizon 4s using the speed test app. We both had 5 bars, no cases or anything, put same place on table and did tests with servers in many different cities.
Every single time his phone download speed would range between .25 MB/s and 1.5MB/s, and mine would be anywhere from 6MB/s to 11.5MB/s. I will say that there was many times his Verizon speeds would actually be faster on the upload then the download speed. Which I though was strange. My average upload was right around 5MB/s, and his was always under one half MB.
I know this is not any sort of technical test but in the real world setting where for once we both had the same signal my AT&T phone slaughtered his. I have always preferred AT&T in general but I have nothing against Verizon in anyway. Like I told my brother in law, if the phone does what you need it to do on a daily basis, the speeds don't matter. Unless the speeds become a bother in a real world setting who cares? He said he can't say he's ever noticed an issue, although its all he's had in the data world so he has nothing to compare it to.
I get very good AT&T coverage in my area in Florida, I dropped calls on my iPhone 4 here and there, have never dropped one on my 4s.
Good luck to you!