Seal of health or Seal of armor for support?

health seals plus armor quints works pretty well for most supports including Leona

Depends on your play style. Armor helps well with early engages(Armor scales well with your W). Health helps well for Relic shield(Shield your teammate base on a percentage of your health). It also depends on who you are playing against. Some champs you get health if they rely of true damage, armor if they don't.

For champions like Leona, Taric, and Braum, I use: Health Regen Quints, Scaling Armor Seals, Flat Armor Marks, and MR Glyphs.

I find that you buy a lot of early health as a tank support between sightstone and Face of the Mountain, so early armor is quite valuable. Though, if you are using Flat Armor Quints and Marks, then Health Seals are a good choice.