Flash Heal is superior. It heals for more (more as in it heals the full amount immediately and doesn't require any time after the fact), is quicker (to apply the full amount of healing to a target), and is perfect to use in between your CoH/PoM rotation. A lot of the damage in raids requires immediate attention, leaving people low is going to get them killed. HoTs are of minimal use in this scenario, when you have a fast nuke heal that will heal them now. Combine this with the heals from your other healers, and the person is full again.
Also because any renew is going to be made useless by the stupid amounts of immediate healing going around from other classes ... which can be mitigated by using mods that allow you to see incoming heals to targets, but still. Flash heal mate, all the way.
Until they change the healing in WoW to not require so many spammy heals, this will be the case.