For Quick Strike, numbers, then durability are the biggest factor i think. The few times I've played, my sturdier designs held up longer, even to the more powerful stuff (because we use dice, having all that firepower's worthless if you wiff... one of the drawbacks of the all-or-nothing system; although, it really speeds up the game and is an acceptable sacrifice) I've never ventured a Clan vs IS game and I think it really depends on the honor rules in use. The more units you can use to tag stuff, the more damage it takes (and dies.) So having more units gets you more damage/shots in on something.
When the games an equal number of units (in the first CSO Quick Strike game at GenCon 09, we each had a light, medium, heavy, and assault) durability usually wins out, although at the end of that game I had the SSRM-6 Daikyu jumping from terrain to terrain keeping up high to hit numbers and killing off 'Mechs left and right. I had one of the last 'mechs on the field and I also lost the first when turn one, my Owens got blown out by something big, bad, and Davion. It takes a little getting used to, but I think it keeps the flavor well enough and adds a different element to the strategic outlook.
