Tracfone uses multiple towers and varies by location
They have contracts with all the major carriers to use their towers, and which one you end up with depends on your location and the phone you use.
There are two main cell phone/network types, those that use GSM (this includes ATT, T-Moble) and those that use CDMA (Verizon, US Cellular, and Alltel). So some phones work on GSM, others CDMA - Tracfone and Net10 offer both.
That's part of the reason they ask you for your location on their website, so they can determine which network would be the best, and from that determine which phones you can use. Its good to check their websites first, if you're going to buy from a store, so you can figure out which one you should get. It may not be a big deal if you live/work in a location with many towers of different carries - you're likely to find one that works regardless. However if you live out in the country, then it can definitely mean the difference between getting a signal or not.
As far as Verizon's prepaid, I had considered it at one point, because with it they do offer a plan where you get free calls to other Verizon users (since I use this mostly at work, and most of them use Verizon...) However I decided not to, as they still either wanted to charge daily access fees (which I'm sorry, but any cell company charging 'access fees' is bull) OR you had to pay a higher per min rate. Even with the free calls to other Verizon users, the 'access fee' would add up to be more than using a per minute rate from a prepaid phone company like VirginMobile, Tracfone/Net10, etc
As an aside to the Verizon prepaid, another option out there for those reading this thread, especially if you want cheap access and are a heavy user (lot of talking, texting, browsing) you may want to check into Straight Talk. It is managed by the same company that owns Tracfone and Net10 (so yes, same issues with customer service problems if you ever need it), and it uses the Verizon network. They offer a $30 month plan (1000 each of mins and text, as well as a set gig of web) or a unlimited $45 month plan.