In the long run: Druids are great because they're designed to stack HoTs on a tank that's constantly taking damage. This is EXCELLENT with a prot pally who needs constant healing in order to get mana.

Shaman are also acceptable healers due to their totem buffs (which all have synergy with paladins). Their single target heals aren't great, but chain heal more than makes up for it.

Considering that you already have a paladin in this group (DI for wipe protection and a hard rez), there's no downside to choosing a druid over a shaman. So I'd say pick whatever class you find most interesting. If I were in your position, I'd pick druid due to the instant-cast hots and considering that a shaman's Earth Shield in the resto tree does not give a prot paladin any mana (since it counts as the paladin healing himself).

Anyway, with 3 characters you'll find instancing is hard, but not impossible if you outlevel the instance a bit. However, with a tank and a healer you'll probably find alot of friends who wouldn't mind filling in those last 2 dps spots.