Ifrit is easier due to ifrit basically telling you what hes going to do next. He is very predictable and the fight is cake. To make it even easier just get a good tank, a couple cons to keep up tank heals and buffs and just run it with a group of thms to burn him down fast.

Moogle tanks a little more teamwork and you need to really pay attn to your surroundings. You need to be sure you are fighting the right moogle and you need to make sure you are not getting hit by the aoes or generating hate from the others. While in Ifrit if someone is a complete moron who doesnt know their job or want to pay attn to when they should or shouldnt cast/ws you can leave a couple ppl dead if need be (except the tank of course), in moogle it really takes the team.

Outside that the Ifrit weapons are better in both appearance and function imo so the rewards are something to consider too.