A lot of the time its preference of leaf over pellets or vice versa. Depending on your setup, sometimes either leaf or pellets can be easier, but as hops, they're fairly interchangeable. There are a few differences to keep in mind though. For one, if you're using these as bittering hops (rather than flavor/aroma/dry hops) then you'll get a better utilization with pellet hops than leaf, since it takes longer for the wort to soak into the leaf hops. If that's what the recipe calls for, make sure you check the predicted IBUs of the leaf hops and calculate the right amount of pellet hops to use. Otherwise, for flavor/aroma/dry hopping, you can just use the same amount of pellet hops as it calls for leaf hops. Also, depending on how much the recipe calls for, leaf hops will tend to soak up more wort than pellet hops. I'm not sure if there's a good way to estimate how much, but unless you're using several ounces of pellet hops in place of leaf, I wouldn't worry too much about it, just keep it in mind if you wind up with a little extra wort at the end of the day.
As far as the differences between leaf and pellets, pellets are just leaf hops that have been ground and pelletized. There are some arguments that some of the volatile oils get destroyed in the pelletization process so some people prefer leaf for aroma and dry hops, but you can also just include a touch more pellets if you're worried. If your system is set up for it, you can also use leaf hops as a trub filter after boiling to remove some of the trub, whereas in my case leaf hops would just clog my pump so I prefer pellets. I do use leaf hops when I'm dryhopping in a keg, but that's more to keep hop debris levels down, and not necessarily because one is better than the other.
