Heartsong is 200spi for 15sec, 20sec ICD, 25% proc rate
Power Torrent is 500int for 12sec, 45sec ICD, 35% proc rate
Average 1.5sec between casts, then heartsong activates on average every 20 + (1.5/.25) - 0.75 = 25.25 seconds
Then heartsong averages 200spi *(15sec/25.25sec) = 118.8 spirit
Power torrent activates on average every 45 + (1.5/.35) - 0.75 = 48.53sec
Then power torrent averages 500int * 12sec/48.53sec = 123.6 int.
And int EP > spirit EP for pretty much every spec of every class that uses int (and isn't using spirit to get hitcapped) It's why they don't let you reforge stuff into int. Power torrent is better, no question.
I wouldn't be so sure about that. I ran a model of full 346 gear, few gems (normal blue amount), all gemmed Intellect for now, and proper enchants minus the weapon.
For the model, the base raid-buffed MP5 in combat was 3446. With Heartsong, using your uptime numbers, it works out to about 75 MP5 over the course of the fight (126 bonus MP5 while it's up). With Power Torrent, it works out to about 27.5 MP5 over the course of the fight (111 bonus MP5 while it's up).
The same model, with 422 gear and full sockets all gemmed intellect, came out with a 101.5 MP5 increase over the fight with Heartsong and about 37 MP5 for Power Torrent.
In the former case, you're weighing 47.5 MP5 against about 578 spell power for the period that power torrent is up (or about 143 spell power average over the course of the fight). The latter is going to be a bit closer on the MP5 side of things.
Not saying you're wrong, but it's definitely worth looking at as an option if the ICD is 20 seconds (I didn't know it had a lower ICD, so I was writing it off as crap, before, personally).
PS: This IS with full mana buffs and replenishment/might. As far as I know I didn't leave any out.