The Overkill for sure. I say this on the authority of having both.
The XM-18 is more overbuilt, has swappable scales, feels better in hand, and has stonewashed titanium(you don't see much of those). And yet, despite all that, the Overkill fulfills the basic function of a knife better, and that is the ability to cut. Not saying that the XM-18
can't cut, just that it wouldn't be my first choice for cutting say, thick pieces of cardboard. I'd want a tool that doesn't necessitate me using both hands to muscle the knife through the material I'm cutting. The next thing is of course, the knife's performance as a flipper. The XM-18 requires a "technique" in order to flip it open, typically by pushing against the blade protrusion up first, and then rolling downwards to generate the force necessary to open it. That actually takes a little practice to get right, and wouldn't be my choice of opening under stress. This is because the knife doesn't have a strong detent(mine doesn't seem to have any detent at all).
If you're primarily after a high-end
flipper, I wouldn't look to the XM-18 for that. The XM-18 is primarily a hard use tool(I don't want to call it a prying tool, but the thick tip and blade grind would suggest it). Since you're in Toronto, why not give Brian Tighe a look?
P.S.
I've got the S110V version:thumbup:.