I'm in an area that's very rural, but somehow has AT&T's new HSPA+ service. I just ran a speed test on my iPad 1, 6.22mbps down, 1.83mbps up. The latency is also fantastic for cellular data, something that REALLY changed when going from "3G" to "3.5G" if you will (to HSPA+). My internet went down the other day and, for grins, I tethered my iPad to my desktop to play some Battlefield: Bad Company 2 (Just to see how bad it would be). I had ventrillo open, which had a ping of about 45ms, and I was playing BC2 with a ping of about 180, with the lowest pings on this server being around 100 (95 or so), and as high as 300. I was actually doing pretty good, some mild lag spikes but all in all it wasn't bad. Definitely the most impressive feature of HSPA+, not that it matters or is practical for the majority of uses, but it proves how powerful that technology is.
SO, that said, find out if AT&T offers HSPA+ in your area, if they do go with it and don't look back! I can still get over 2mbps down in areas where I have no "bars" at all, I never drop calls or anything, it's really fantastic (the way AT&T SHOULD be for the price you pay!).
Verizon is pretty much gonna be slower than AT&T in most places, BUT, no connection is slower than a slow connection, LOL. I'd look at your area and the places you will be, and then determine who offers you a connection period, from there you can decide who has the best features. If AT&T does NOT offer HSPA+ in your area, there is a good chance they won't, they will probably skip it and go to 4G (which makes it useless for your 3G iPad, you'll still be on 3G until you upgrade to the iPad 42.6 or whatever), if that's the case speeds between VZW and AT&T are probably going to be similar, maybe 1/4 to 1/2 of a megabit faster for AT&T judging on most reviews, and that might not be worth it if they are more expensive, or it might be, that's up to you!
But, again, if they have HSPA+ I'd definitely do it, it's the fastest a current generation iPad can possibly have with mobile broadband, short of 4G which is not a feature of the current iPad! Theoretically, HSPA+ is as fast as the fastest the iPad/iPad 2's 3G chip can handle.