Short Answer: Troll is the better shaman race.
More involved answer: Trolls and Iksars both have regen, which is awesome for Shaman (although I think most would agree that in the long run Ogre stun immunity is more awesome). Both are disliked by any good race. Both have the same (large) racial experience penalty.
However:
- Trolls can wear plate. First there's the famous JBB: I remember it being essential Shaman gear back on live, but here I think the consensus is that it's only really good if you can afford to get one to level up with. At 60, its value is a lot less clear.
However, Trolls also get to wear the other great Jaundiced pieces. These can save you a ton of mana on raids because they have right-click stat buffs, and most are very affordable (<1k). Finally there's the chest piece (NOT so affordable) which offers one of the only ways for a Shaman to get a damage shield.
- Trolls are not hated by Dark Elves, other Trolls, Ogres, and The Overthere. EVERYONE hates Iksar, but at least Trolls have a few different cities they can sell/bank at.
What do Iksar have against Trolls? A slight racial AC bonus, and they aren't hated in Cabilis.