Using the mana drain Javelin is a little better since you still get the patk bonuses from the tree ontop of it's better base power, and, more importantly...
Since the point is MP drain, it's something you'd switch on for a battle or two, and then take back off for a real weapon. Using a Javelin means you don't have to rearrange your POW trees each time, as you otherwise would in order to get Stave's MP Drain pow tree bonuses. It also means access to the good cheap Giant Throw skill for AoE and more damage and such, with a super low MP cost instead of one of Remi's non-weapon skills.
So, it's much more convenient.
A staff is still alright for a plan based on Red Magic and Remi's HP attack skills (Life Steal, Soul Steal, Star of David, etc, but it's a rather dubious idea due to cooldowns and other reasons), or if you don't want to/can't make the mp draining javelin. The javelin isn't a TON better. But it's enough to warrant crafting it.