advice wanted about soul drain skill

[fruithead]

i am playing a mage, right now i have 40 rage and lvl 1 rage skill (so i could potentially have another 20 rage) i am being offered the skill choice to improve soul drain to 40% which brings the price to 45 rage. after the second reduction in rage cost it will cost 35 rage. even if i don't take it now, i am wondering if it's ever going to be worth taking. even though 35 rage for 40% is a good value, right now i am at 30% for 20 rage, and i am able to consistently use soul drain in battles without a problem, despite the ridiculously slow speed at which mages gain rage. in fact, i almost always have enough rage left over from previous battles to open up with a soul drain right away.

although i think it's obvious that a mage will never be able to deal with the 50% stack damage rage cost, i still wonder, is it really worth the extra 10% stack damage to lock myself in to a 35 rage cost (and that's presuming i get offered the reduction, although i assume i will) for the rest of the game? also, part of my concern is that soul drain is obviously most effective at the beginning of a battle.

another point for consideration: is the xtra 10% good enough to warrant not being able to toss around this skill at will? now i almost always have enough rage to use it twice per battle IN ADDITION to any other rage skills i care to use. (i kind of like ice ball and gizmo) but an extra 15 rage cost would mean i will have to be a lot more frugal with my rage spending.

any help is appreciated!!

thank you!