I would stay away from subsidy plans, especially T-Mobile ones. You can get the AT&T version of SGS2 used on ebay for ~$350 and go prepaid. One of the easiest phones to unlock should you still want to use it on a T-Mobile prepaid plan. It's a better phone than anything T-Mobile has to offer and likely the first Gingerbread native phone to get ICS.
If you insist on a T-Mobile phone for data speeds, they'll let you pay $200 for a new phone and the rest you'll be paying off $10 per month for 2 years.
Buying a used SGS2 and going to Straight Talk is your best option IMO.
SGS3 will likely follow the predecessor's schedule, meaning a international version is likely to be launched in May with a high price tag at launch. It's doubtful it will be compatible with T-Mobile's HSPA network. If you don't have frequent wifi coverage and data speeds are important to you, stay away from T-Mobile.