At lvl 80:

Healing Wave Rank 14
25% of base mana 40 yd range

3 sec cast
Heals a friendly target for 3034 to 3466.

vs

Greater Heal Rank 9
32% of base mana 40 yd range

3 sec cast
A slow casting spell that heals a single target for 3950 to 4590.

Questions:
1. Do they scale equally, without talents? I assume they do, but assumptions kill.
2. What is the base mana of a lvl 80 priest vs a lvl 80 shaman? How is the mana efficiency comparison without talents? I assume Greater Heal wins, but I also assume shaman base mana is lower than a priest, thus it won't be by such an insanely amount as it might look on first glance.

From what I see, Holy Light beats both anyway.