A few weeks ago I was able to nab Twilight's Hammer, and I did a mad dash for the enchanters. I was quick to slap Power Torrent on it, as I'd been running with Heartsong on my weapon up to this point. I mean, Power Torrent is an epic enchant, with epic mats - it HAS to be better - right?
Unfortunately, the math doesn't seem to pan out. Here's what I've been able to put together so far. For the mp5 numbers I used this formula - .0025 + SPI * sqrt(INT) * .0083625
Awarding 200 Spirt for 15 sec, this enchant has a 20sec ICD and results in about a 57% uptime. Right now this means I'm picking up an additional 121 mp5 every time it procs. Over the course of a five minute fight, this averages out to about 4356 additional mana. Figure that each proc will give you 3 ticks of your mp5 boost.
Awarding 500 INT for 12 sec, this enchant has a 45 sec ICD and results in about a 17% uptime. Plugging the additional INT into the formula above, and adding the returns from replenishment, this averages out to about an additional 185 mp5 for me right now. With the INT bonus only lasting 12 seconds, you only get 2.4 ticks of the boost.
What isn't factored here is the mp5 you would get if you hit Divine Plea while PT is up, or the additional spellpower you recieve from the INT.
I've never had a tank die becuase the heal that did land wasn't big enough by a thousand or so points. I have lost tanks because I've run out of gas though, and that's the final deciding factor for me. If Power Torrent gave your current mana the boost that your max mana recieved, then Power Torrent would win in a landslide - in fact I'd say that it would almost be borderline OP. Since it doesn't though, you're really just pushing your top end up.
At the end of the day, it doesn't seem worth the cost to put Power Torrent on. Your total mana return is going to be higher with Heartsong, you'll pull in about double what you will with PT. You might see some slightly larger heals during those 12 seconds, but unless you're able to time your other CDs with it, you won't see noticible increases. For now, I say leave PT for the DPSers, and stick to the regen.