It's personal preference.

The pro of live is that it's natural and some owners prefer it, the con is that sometimes the rat can scratch/bite the snake in an awkward constriction and leave minor-serious damage; although usually minor. Another thing is that buying live one by one can run up the bill in the long run unless you plan on breeding your own rats/feeders.

The pro of F/T is that you can buy several at a time, select preferred size each time and never have to leave the house. It may be a bit more time consuming since you have to let them thaw over the course of a few hours or defrost them in a bag submerged in hot water.

In conclusion, it's up to you.