If you choose the regular StraightTalk plan, you will have excellent reception off the towers that are near you. I have been told that mostly Verizon's towers are used, but since StraightTalk is a partner or service of TracPhone, it uses whichever towers you are near. So, no worries, and no towers in particular. I can tell you I have better reception than my sister's AT&T and my other sister's Boost Mobile.
The Straight Talk Samsung Galaxy S II will run off of AT&T but this phone, the Galaxy S III, will run off Sprint.
Could the Walmart associate have thought you meant the S II?