You should get the iPhone 4 with whatever carrier gives you better coverage in your area. Verizon may have the best overall coverage, but it may not have the coverage you need. AT&T is also a bit cheaper than Verizon, but like I said, Verizon has better overall coverage.
I would go with verizon if I could afford it bc the unlimited data plan is attractive, especially if you want to use your laptop on the go by tethering to your iPhone. Plus, they already have 4g. If Tmobile, however, were to get the iPhone I'd be with them in the blink of an eye bc they offer the most for the least expensive. Sadly, I'm stuck with AT&T for now due to income restrictions.
AT&T does voice & data at same time on iPhone a lot of people dot realize how much it's useful. Verizon doesn't do it and has plans to do it on their lte but iPhone4 won't support it. I like to know I can send mms to someone while talking on the phone or browse Facebook or Internet while on 3G instead of turning wifi on and killing the battery more.