Congrats! So jealous.
I'm surprised that Tmo didn't at least give you a month or so of free service. If you can get the phone unlocked, it can be used on AT&T's HSPA+ network. The one thing I liked about the Tmo variant was the additional HSPA+ 1900 MHz band, which is what AT&T uses, so it is the most universal GSM/HSPA+ S3 model! You will get HSPA+ on Tmo, Wind, Rogers, and AT&T (as well as overseas).
Only got the AT&T version because my company pays for the phone and service, so I can't complain either.
Nexus 5 - Stock, unrooted (AT&T)
Samsung Galaxy S III (SGH-I747 d2att) - CM 10.2 (T-Mobile)
Asus Nexus 7 WIFI (2013) - Stock unrooted
iPad Air LTE - Stock (T-Mobile)
My YouTube Channel