I would have to say Heartsong for the little extra mana regen, Powerturrent is a proc that gives spellpower(intellect) and yes its still a valid enchant for resto druids. But when it wears off you have an internal cd on it(ima guess 45sec) So the "100% uptime 200 spirit enchant" will be victorious!
Power torrent is resto druid BiS.
Power torrent equals out to 120int when talking about icd.
Heartsong equals out to 108 spi.
Since replen, innervate and revit are all effected by int, not spi, the regen difference between the 2 is very small. Int also buffs SP and crit, putting power torrent for resto druids well above heartsong.
Unfortunally, power torrent is still a very expensive enchant, and to my knowledge, only those who are well into raiding are using it at the moment.
Using it for heroic weapons would definitely be overkill.
I'm using power torrent at the moment and it's the best weapon enchant for resto. However, don't bother to do it before you have a weapon with less than 359 item level, it's not worth the investment with the current prices.
I'm using Heartsong. I'm ilvl 340 so can use the extra spirit regen. I think it's really situational and gear dependant. Our raid resto druid is using heartsong for now, until he upgrades his wpn to make it worth the power torrent cost.