I try to focus on setting up then maintaining. I'll start a battle off by trying to tag as many enemies with purifying light (deals 33% all damage taken, leaves a small healing field) then I combat prayer everyone for the buffs, then I use rapid regen and healing springs to keep everyone up... Breath of life is my "oh s&%$#" heal, i actually hate using it and try to use as little as possible, but if someone dips below 75% health ill bust it, depending on the mob/boss/team ill even let that dip to 50%... I prefer to focus on HoTs>bursts.

I generally run Rapid Regeneration, Illustrious Healing, and Breath of Life for my big heals.

I then take Jesus Beam, Puncturing Sweeps, and Soul Shred for damage

For support I take Cleanse and Repentance and War Horn.

For my personal defense, I run Annulment.

I then have two flex slots which are currently filled with Vigor and Siege Shield.

Right now I am using purifying ritual, healing ritual, and breath of life. I mostly pvp but I also do dungeons and probably will eventually do raids

I like using both normally I have a dps bar with 1-2 heals on it for burst purposes and I have a sustain bar. I love Hot heals because they let you do something else like throw shards and do some damage. i will tend to put on a hot heal in the beginning before running into a group of enemies be it pvp or dungeon if hp drops low i use the resto shield( i have morph that always gives myself a shield + the lowest health person) then burst them back up in hp and throw on a hot heal and work on getting back some magicka and deal damage and rinse and repeat.

I would suggest not to focus on any specific type.
You do want to develop the entire rejuv staff and all class skills of the Templar.

Each encounter and group should determine whats on your bar so don't limit yourself.
Play around with a mix of DPS that does HOT, but always be able to have one OH SNAP heal ready, but think about being able to sustain long periods of burst as well as long periods of HOT stacking buffs and or reducing dmg or elemental dmg.

It also can help to offer passives stamina, magic boosts to the group.

What I'm saying is don't build yourself to be linear, balance yourself so your only worry is knowing the fights so you can adjust as needed.

Personally I like to be everything in one and adaptable healer. Mainly because I like to carry pugs through dungeons when I need to and help out noobies like I was.

Breath of life for burst healing.
Purifying ritual for sustained and allowing allies and yourself to cleanse whenever they need to.
Repentance for group/self healing at massive amounts and group stamina.
Spears for more group stamina.
Channeled focus for magicka sustain + spell/physical resists.
Remembrance for the OMG*******WEALLGUNADIE!!1!!!111 moments...emergency wipe heal.

Then throw in some damage abilities and finish skills like puncturing sweeps and radiant oppression.

Then throw all CP into increased crit damage and heals, increased healing, increased spell pen, reduced cost of spells, increased magicka recovery, increased magicka return on heavy attacks, reduced stamina costs of break free, dodge roll and reduced effect of fears etc, increased spell resistance, increased elemental damage resistance, reduced poison desease magic damage done to you, get the resurrection passives.

Throw in some spell damage / regen potions and some health + magicka total food and your good to go.

While leveling 5 set seducers + 5 set warlocks + 2 set torugs pact + staff that increases enchant with magicka absorbing works pretty well.

Once max level upgrade to whatever you feel like will work with your group healer build.

The trick to being an awesome healer is staying alive, never running out of stamina for group or magicka for yourself, you are the battery of resources for group not just heals and still be able to do some decent damage, finish stuff off quickly and maybe even tank a little if you need to in an emergency, oh and be able to ress people fast.

It's not as easy as people think, if you get a bad pug group you could end up carrying everyone and having to be main healer, tank, damage dealer and support all at the same time...

Oh and always makes sure people have at least 15-17k health when starting if in pugs or they will get one shot and not get heals. I usually hand out health food at start of dungeon to make sure everyone has enough so my heals can actually catch them if they screw up.