Best Individual Plan for an iPhone?

I need some advice from someone who knows about iPhones/phone plans. I'm 18 and I'm looking into updating from my Blackberry to an iPhone. Right now I have Virgin Mobile and I don't like it other than the fact that it's cheap and I have to be on a plan alone, so it seemed like the best bet but the service is horrible. For the iPhone, I'm either considering Sprint, AT&T or Virgin Mobile again. Since I have to be on the plan alone and am leaving for college in a few weeks, I wanted something that I could afford considering I'm leaving my job. The Virgin Mobile plan is unlimited messaging/data for $35 a month, but the phone has an upfront cost of $649.99. The AT&T plan that I'm looking at would be $39.99 a month, unlimited rollover minutes, 5000 unlimited nights/weekends, and unlimited mobile to mobile - it sounds like it would work for me but I'm just wondering if it's likely that I will go over my data if I'm on the internet a lot? The sprint plan is 69.99 a month for unlimited everything pretty much. I don't know which one would be the best bet for me considering I'll be in college. I would like to avoid virgin mobile if I can but if it is the best bet I could consider it. Thanks for any help!

Added (1). Oh and I RARELY talk on the phone so minutes aren't a big deal

At&t is NOT $39.99
thats just the voice plan
you will be required to also add a data plan which starts at $20 a month, so you would be looking at a starting plan with AT&T at $59.99.

Sprint is also not $69.99 since the iPhone is considered an "advanced device" it requires another $10 a month fee, so its actually $79.99 a month.

Virgin Mobile uses Sprint towers for service.

I would wait until you get to college before you get a new cell phone plan. This way you will be able to see what provider works the best in the area that you are going to be living in.