I would recommend finding a way to test T-mobile in your various scenarios before committing to making the switch. Unfortunately, it seems like you may need to pay the ETF before you can make the evaluation. Is there any way you can convince ATT to unlock your phone in advance, so you can try out T-mobile? If you are a long standing customer, maybe you can convince them....try telling them you have an international trip planned and you need your phone unlocked. If you can get your phone unlocked, I'd recommend one of the following strategies.
1.) Get 2 T-mobile SIM starter kits. I think it's possible to get them for free now if you use the code "GETASIM" on checkout. The site is:
http://prepaid-phones.t-mobile.com/sim-card
Activate one of the cards in your unlocked phone with a new number. I think it will have $3.34 of credit on it. Use this to buy one day of the unlimited $3 a day plan for voice, text and 4G data (up to 200MB before falling back to 2G). You will have 1 day to try all of your different use points to see if the service is sufficient. You can also recharge with as little as $10 if you want to try it a couple of more days before committing.
If you like the service, use the second SIM card to activate on the $30 plan (you won't be able to change the service on the first SIM over to this plan as it is for new signups only). Here you can choose either a new number or can choose to port your ATT number which will automatically cancel your ATT service and then they will hit you with the ETF.
2.) You can do essentially the same as in 1 above but just get one SIM and activate it on the $30 plan with a new number. Do all of the testing as above....you will have up to a full month to evaluate service. If you like the service you can call T-mobile at any time during that first month and tell them you want to port your ATT number over....that will discard the new number you got and then your T-mobile service will have your old ATT number.
The first way allows you to risk less than $30 to try out the service. If you want longer to evaluate then the second way is probably easier. In any case, try to port your number over near the end of your ATT billing cycle. ATT will not give any kind of prorated refund for a partial month, so if you cut things off even one day into a new billing cycle, they will charge you for the full cycle.
When did you get your iPhone 5? If it was prior to April 12, 2012, then it doesn't support T-mobile's 3G/4G bands unless they are refarmed. The good news is, if you are mostly in areas that have LTE, then you are in mostly refarmed areas. So your phone will work fine. I think you mentioned that you are in NYC, so it shouldn't be a problem. Just be aware that if you venture to non-LTE/non-refarmed areas, your phone will drop to Edge, even if T-mobile has 3G/4G service in those areas. If your iP5 was bought after April 2012, then it will work with all of T-mobile's frequencies and you will get 3G/4G in more areas.
The phone will show LTE when on LTE, 4G when on HSPA+, 3G when on HSPA, and E when on Edge.
Again the most difficult thing for you will be getting your iPhone unlocked to try out T-mobile. Or you could just find a friend/colleague/stranger who has T-mobile on an iPhone and try to find out what their experience has been in your area. It really doesn't do a lot of good to ask in general as results are really specific to where you use your phone.
Good Luck!
