well, I've only had T-Mobile for less than 48 hours.... and it's a few dollars more, but in my area at least (Louisville, KY), Sprint doesn't have 4G and T-Mobile does. It's not as fast as AT&T or Verizon here, but I'm still getting 10 meg down, whereas on Sprint i couldn't even get 1 meg......
and for me at least, the way i have my plan set up, it's literally a $2 monthly difference... i dont have Unlimited Data with T-Mobile, I went for the 2 GB plan to see how it works out for me, but I can bump it up for $10 more. With Sprint I did have Unlimited Data, but with dial up speeds here, unlimited data is pretty useless to me...
Plus I got the new customer price on the SGN2 with Tmo, and I would have the upgrade price on Sprint, so with my company discount, I ended up getting the phone cheaper