Depends, honestly. If you're the sort (like me) to stack health and spell power over mana, then +150 health might be your best bet. Especially when your other gear (if you're in caster gear) will have a lot of intellect, you may find that your mana is plentiful, but your health is a little short. In those cases, you may find that +150 health increases your survivability. If you're okay on health, but low on mana, then go +6 stats.
Another underrated enchant that doesn't get talked about a lot is +6mp5. You really need fishing hat and AGM to be able to do this effectively, but if you can pull it off, you can increase the duration of your mana pool by a lot. I have yet to see a person do this effectively, but it's potentially a very powerful build. (To do this, you really need to stack it with BoA mace / Advisor's Staff, +5mp5 on bracers, and Advisor's Ring so you can stack mp5 gear. I think you can get up to 18ish at level 19, which will make your mana regen MUCH more quickly than those around you.)