Should I buy a iPhone 4, 4s, or 5 and get it unlocked?

I want to buy a iPhone 4, 4s, or 5 but I'm with T-Mobile so I would have to get the unlocked service the unlocked costs alot so i was wondering is it cheaper to get the phone then take it to get unlocked or eBay the unlocked one. Also if i buy the phone then get it unlocked what service should i get at&t, sprint, or verizon. *I'm on a budget

Added (1). When will T-Mobile get the iphone?

Well your in a hole. It's hard to get a unlocked good iPhone. Apple barley makes unlocked iPhones. With carriers I would actually choose boost if your getting a unlocked iPhone. Abuse the $55 bucks a month that vanishes all the way to $35!

TMobile is getting iphone in 2013, I would wait and get it then. No messing around with unlocking and you can then get it discounted on a contract.

T-Mobile Unlock means GSM unlock. It lets you use any GSM SIM card in your phone. In the USA the only GSM carriers are T-Mobile and AT&T. If you take the back cover off of your phone there's a llittle card in it. You want to be able to take the little card out of your phone and put in another. That is SIM Unlock. Don't buy a Sprint or Verizon phone which run on CDMA and expect it to work on T-Mobile GSM network. They might not support GSM. You might consider getting a real Android phone. I've had a few of both. IPhones work great, but it's like having training wheels you can't take off. Android lets you be free.

You should switch to AT&T and get the iPhone 5.It is $200 while unlocked is like $600.