I use T-Mobile and don't have any problems with the signal strength. But as with all providers, it all depends on the exact spot. You can check T-Mobile coverage for any particular address/ZIP here: http://www.t-mobile.com/coverage/pcc.aspx
Don't worry about cell strength as long as you have home wifi. TMobile allows you to place voice call over your local wifi network for free on most of their smartphones--including the new Samsung SIII. http://t-mobile-coverage.t-mobile.com/4g-wireless-broadband-service