Little bit more to it than that, by using a pump action marker you will find by default that you will use less paint (cos you will be unable to chuck as many as a semi, under ideal conditions of course
) When using a pump action marker you will find that more of your upper body tends to be exposed so thats a down side to them. go with what you feel you can afford...sites will also tend to allow a pump marker on its site when you have "normal" punters there due to the two factors above, when owning a semi will stop you using the marker at sites unless it is either a walk-on day or a tournament.
I own several of both and very very occasionally get to use the pump marker (tend to use it on the very rare times that there is a pump only tournament, 'cos to use a pump in a "normal" tournament these days I would be toasted very quick)
semi's (generally) are accuracy thru volume, pumps are as accurate as you feel them to be (some swear they are more accurate) make your own choice.