I would suggest the shield skill--shields are extremely helpful when you're on horseback. If and when your horse gets stalled you can turtle up and, hopefully, deflect incoming attacks while you wait for the horse make its escape. Using just a 2h weapon, you would need to manually block those attacks, which would suck--a shield, however, BLOCKS ALL! A shield also protects you against missile attacks, which is helpful both when you are stalled out, and when you're on the move. Think escaping from horse archers.
That shield may also come in handy on infantry maps. Many two handers carry a cheap shield around in case they need to survive some ranged bundle of sticksry.
Of course, you can function perfectly well without a shield, particularly when fighting other cav... the down block is magical, truly. When dismounted, your extra IF/athletics might come in handy.
It's up to you, really. It comes down to whether you would prefer increased survivability on horseback, or better performance in melee combat.