It's all relative really. Breton healers are great, but Altmers, Dunmers, Imperials are also excellent. I have 2 Bretons Templar and 1 Dunmer Templar and I must say, my Dunmer Templar is better in everything else than my Bretons, except pure healing.
Templars healers need to block and tank sometimes (last boss in VDSA), and the 6% stamina helps a lot with my Dunmer in so may ways. Also help roll dodge (Manticora, archers in VDSA, etc.) and PVP (even with magicka builds, you need an enough amount of stamina there).
Second, templars healers use a magicka build and when you seek a better time in a weekly or just help your team, you need to do dps as much as heal. Again, my templar dunmer exceeds my bretan templars. First, in magicka builds, the better way to help in dps is with a Inferno staff (your AoE will be Impulse, you need to aply Elemental Drain, Crushing Shock to interrupt and make range dps, etc.) and Dunmers have 7% more damage. Second, Vampire Bane is the best DoT for magicka dps templars, and it's a flame damage based skill.
This means the Dunmer Templar Healer is better than the Breton Templar Healer. Not really... Just means that using, my favorite armor sets (5 Healer + 4 Magnus + 2 Torug; or 5 Healer + 5 Eyes of Mara + Master Weapons), with the same build, same rotation, etc., I think I have a easier life with my Dunmer Templar than with my Breton.