Healing meters should only come into count if your either;
A, not clearing a boss due to healer issues
B, dropping a healer to get another dps
In either case, it'll normally come down to the CLASS, and not the healer. Versatility is the key here, i would rather take a resto druid over a disc priest based on this alone.
Although cause rarely are 2 people the exact same skill levels, lets break it down.
If someone is going out of mana, and its due to using wrong spells at wrong times etc. Not cause dps/tank are taking extra damage due to stupidity. Have a look at the overhealing (and healing done, not raw healing done), and decide accordingly how well each person is doing.
If everyone is fine mana wise, you go the versatility, group buff and healing meter route. Obviously if you've already got every buff covered (normally a 25 man), that doesn't become an isssue.