I would go for the paladin as it makes instances more accessible. It also makes a great human shield for PvP (since they mostly attack your main character, which I assume is going to be that paladin) and he provides very useful buffs for your shamans.

Cons:
- having to re-level herbing/skinning (these are the useful professions on the shaman that would have to be dropped from the group). Or perhaps take blacksmithing instead of one of those professions, in which case I'm a gathering profession down...
Herbalism and skinning are VERY easy to level. I would actually recommend herbalism+alchemy if you don't have alchemy yet. Blacksmithing is VERY expensive to level (if you consider the ore that goes in it and what this ore is worth on the market).

- abandoning my "main" shaman :(
Shouldn't be a problem. It's one less shaman that you have to share gear with and you can always switch him back if you like.