And, admittedly going to the way way back machine, the post-paid accounts get cell tower usage priority. Also, I believe some prepaid plans don't utilize all the roaming/other carrier agreement towers. You may not get the exact same coverage as someone on post-paid Verizon. It used to be that way anyway...
That being said (guessed), a friend is on Verizon prepay and has zero issues. He just pays less.