I wonder why noone else has brought up this "issue" yet, especially on the Beta Forums.
At first, here are the Numbers for the LvL 80 Ranks of both spells:
Circle of Healing Rank 7:
Heals up to 5 friendly party or raid members within 15 yards of the target for 684 to 756.
21% of base mana, 40 yd range, Instant cast
Flourish Rank 4:
Heals up to 5 friendly party or raid members within 15 yards of the target for 4410 over 7 sec. The amount healed is applied quickly at first, and slows down as the Flourish reaches its full duration.
32% of base mana, 40 yd range, Instant cast
Not taking any Bonus from Spellpower into Account Flourish already heals for about 6 Times the amount that Circle of Healing does.
That means in order to get the same amount of Healing a Holy Priest would have to spam Circle of Healing 5-6 Times (7.5 to 9 Seconds), putting the Mana cost to at least 3 times as much as that of a single cast of Flourish.
This also means that the Priest is doing nothing else during that time, while the Druid would just cast that one Flourish and could afterwards continue healing the MT.
Of course those numbers are not 100% accurate considering Spellpower Bonuses. But even those would work in favour of the Druid as Circle of Healing only gets 25% of Bonus Spellpower and Flourish as a 7 Second HoT will surely get a much larger Bonus from Spellpower. ( 5 Casts of CoH giving 125% Bonus while a 7 Second HoT will probably have a 100% Bonus ?)
Anyway even if my numbers are a bit off, Flourish still is much more Mana efficient and still gives MORE Healing per Second than pure CoH spam would give. Who in their right mind would use a Priest for Group Heal when there's a Druid around who could do it that much better ?!