My advice would be, if you only have 15-20 snakes and they all eat F/T is to stick with it. To add to what Deborah said,
Pros:
Cheaper (kinda)
Quality (you know the health of your rats)
Live always available (picky eaters)
Convenient (food is always on hand)
Cons:
Time consuming (cleaning)
Expensive to start (racks, watering, etc)
Smelly (omg yes)
Space (racks, food, bedding)
Killing rats is not fun!
Honestly, if I almost wish I didn't breed rats. They're a lot of work and they smell. The convenience factor is the only reason I breed them. When you buy F/T in bulk, the price comes close to even when you factor in food, bedding and racks. Building a rat rack is worth like 80+ rats. For smaller collections that readily eat F/T I'd advise against it. But, once you start getting into feeding 300+ rats a month and you have space/time available, it becomes worth it.