It all depends on the coverage in your particular area.. For example, compass.t-mobile.com will provide precise, down to the block, coverage information as far as T-Mo goes.. Ask around with what your friends have, see if you have a lot of friends with a particular carrier (such as with Verizon, ALL of Verizon's plans have free IN-network (mobile-to-mobile with other verizon phones) calling. And then decide on your new plan.
Take it easy,
jblu