Altmer has more magicka and some weaknesses that could be fun to work around, but I always played Altmer mages in both Oblivion and Morrowind. Maybe it is time to try a Dunmer mage and try to work around the reduced magica. I will probably go with the Atronach birthsign either way, so I won't be particularly short of magicka with either race.
From a roleplaying perspective, a Dunmer seems like it might be more fitting, especially since I want to join the House Telvanni. I am interested in other people's experiences playing these two races in Morrowind. Without spoilers of course.
Edit: Dammit, I just noticed the typo in the title! I am not good with keyboards. Unfortunately I don't think I can fix that now. It was unintentional and if there is a way to fix it, let me know. My appologies to all you Dunmer fans!

I guarantee it's fun to be an Altmer, but maybe you have fed up being one yourself, seeing you say you've played them many times? So, I'd recommend a Dunmer for you. Mana pool won't be much of a problem when you're at higher levels and use alchemy and enchanting to boost it up if needed.