Rollover minutes are REALLY nice, plus AT&T has a better network and 3G coverage (T-mobile just started their 3G network, whereas everyone else is prepping to rollout 4G).
The reason why I would never go to AT&T, however- they charge an arm and a leg for their services, and are less cooperative to their customers than T-Mobile is. They've sort of developed an arrogance ever since the iPhone came their way.
In other words, if/when you need to complain about something, I've always found T-mobile to be very willing to work with me and make sure I'm a happy customer- they care if they keep your business or not. AT&T has started to develop this "you don't like it? then leave" sort of attitude that reminds me of all the things I hate about big corporations like Verizon.
So, it comes down to this- if you're happy with your T-mobile coverage, and only want to switch for budgeting purposes, I think you'll get more for your money sticking with the pink T. A plan with a decent amount of rollover minutes will cost you as much as a plan with more minutes on T-mobile anyway. They know this, which is why they can offer it as a plan perk- it gives the illusion that the customer is saving money.
Food for thought: I had T-mobile for years, and loved them because they made me feel like I was valued as a customer. One time I called them up and asked a question about prorating a service, and out of the blue the rep told me "BTW, I see you are approaching your minute limit for this month- you've been a customer with us for a while, and we'd hate to see you upset with your plan so I'm going to give you 100 minutes for the month so you don't go over. But be careful in the future."
I *didn't* ask for this, or even mention anything about my minutes! I actually knew I was almost out of minutes, but the billing month ended soon after that so I was ok- this is just the sort of thing that doesn't really cost T-mobile much to do, but its important to them that I be a happy customer. That is why I enjoyed giving them my business.
Another time, they offered me a free phone replacement (actually an upgrade to the next model) after my phone had a mechanical error- and I had no insurance or anything on the handset. They didn't charge me or make me resign my contract- they considered it a warranty replacement because they want to make sure I can continue to use their service. I wish all corporations were like T-Mobile!
Good luck trying to get that kind of treatment with AT&T.
PS. I should add that I have switched to Sprint because the coverage is better than T-mobile, and they have fantastic 3G (I use a lot of data). Sprint isn't as nice as T-mobile, but they're still enough of an underdog that they want to keep my business.
