OK, time for a little lesson on data plans. AT&T and Verizon each have three options.

AT&T:
200 MB for $15
2 GB for $25
4 GB with tethering/mobile hotspot for $45

Verizon:
2 GB for $30
5 GB for $50
10 GB for $80
Can add mobile hotspot to any of those for $20 extra, which also gives 2 GB data

Now, while Verizon may have higher plans, they're a helluva lot more expensive, and most people won't need more than the 2 GB plan on either carrier. If you don't need data that much and just want the smartphone, you can pay for the AT&T's smallest plan (200 MB for $15), but with Verizon the least you could pay is $30 for 2 GB.

Me personally, I have the 200 MB plan with AT&T and just use Wi-Fi when I need to. Wi-Fi is everywhere anymore, so it's not hard.

Also, a note, if you get an unlocked iPhone and decide to use it with T-Mobile, you won't be able to use the talk-and-surf feature. That's only available in 3G areas (since your phone would use the GSM radio for talk, while using the WCDMA radio for the data), and AT&T uses a different 3G band than T-Mobile. They both make a point of not supporting each other's 3G bands on their phones.