It also depends on how fast you intend to be shooting... Gravity fed hoppers are limited to around 6-8 balls per second i believe, and also prone to a lot more jams so it develops a sort of "shake and bake" form of shooting
If you're just going to be taking it easy and playing casual games, gravity will be ok but won't be entirely reliable.
Electric fed hoppers do (most of) the work for you, depending on how good the hopper is, and can churn out paint a LOT quicker and more efficiently.
It all boils down to the level of paintball you're going to be playing buddy.