Trying to assert a complete distinction between the two is a fools errand (as Gandalf would say!) as the two things are clearly linked - obviously I see now what you meant by power but I for one have been trying to compare Wallace and Kvsesic in terms of their relative 'strength over the ball' as I believe this is the most important thing for a 7 in addition to general wiliness and ability to slow down the breakdown.
I presume you mean therefore that Kvesic is more powerful in the loose and in defence? Which is all very well but surely the reason we're all chomping at the bit for a genuine openside flanker is to have that extra strong jackler in the side - if we just wanted a 7 who tackles his heart out and carries pretty well, why look any further than Robshaw, whose fate i would argue is that he will be underrated by most for eternity? In other words surely Wallace being stronger over the ball is a better point of difference bearing in mind what the current England set-up lacks? You can criticise Robshaw's jackling ability but very little else imo.
Just my thoughts anyway.
Saw an interesting point somewhere about Kvesic and the fact that in the 2011 junior world championship he played against Sam Cane for New Zealand; Since then, Cane has gone on to get 11 caps for New Zealand despite the stronger competition in the position, whereas our boy Kvesic has only got 2 caps, and those were whilst the first choices were taking a rest.
Don't get me wrong by the way, I'm a big fan of Kvesic generally but I don't see a great deal of evidence that he's much ahead of Wallace(if at all) and in any case it's not like good players cant' shine in a poor team; I don't think Kvesic would be using that in defence of his form either to be honest.