Virgin Mobile VS Boost Mobile

Ive narrowed down a prepaid cell phone to 2 vendors: BOOST Mobile (Nextel) and Virgin Mobile (Sprint PCS). Primarily because both have recharges of 90 days for 20 dollars. I want to know, all else being close among these 2 vendors, what is the network comparison like among these two in greater metropolitan areas, for signal strength, dropped calls, dead spots, coverage area. Both entry level phones for these vendors have pluses and minuses but I am most concerned about stable network coverage. What are your experiences? Thanks.