The trick is to start healing when you *expect* the tank to lose a lot of health in the next 3 seconds. You don't wait until he has low enough life for the heal. This shouldn't be that hard if you know the tank and the enemy he's fighting.
Right now I get about 3.5 points of healing per point of mana with LHW, and 3.9 points of healing per point of mana with healing wave (level 68 resto). That might not sound like a big difference, but over 20,000 points of healing, you save 600 mana, granting an extra healing wave (which for me atm gives ~2.5k). If you factor in crits, I imagine you're gaining a bit more.
One last point to be made - you do not regenerate mana while casting. You will likely find yourself casting more often if you use LHW. Although the healing done per second of casting time is about the same between the two spells, you do not begin regenerating mana for a couple seconds after you finish casting. Since you will have longer between casts with healing wave, you will regenerate more mana.
Healing wave is more mana efficient. QED
Right now I get about 3.5 points of healing per point of mana with LHW, and 3.9 points of healing per point of mana with healing wave (level 68 resto). That might not sound like a big difference, but over 20,000 points of healing, you save 600 mana, granting an extra healing wave (which for me atm gives ~2.5k). If you factor in crits, I imagine you're gaining a bit more.
One last point to be made - you do not regenerate mana while casting. You will likely find yourself casting more often if you use LHW. Although the healing done per second of casting time is about the same between the two spells, you do not begin regenerating mana for a couple seconds after you finish casting. Since you will have longer between casts with healing wave, you will regenerate more mana.
Healing wave is more mana efficient. QED