depends on the rifle in that will the short tube of the 1.75-6 fit between the rings w/o resorting to extensions, etc? if not, then the 6x36 is the choice. if so, the decision is tougher, and really goes beyond the scope of the topic at hand, but, hell, i got a minute and a good hijack once in awhile never hurt nobody...
since we are talking leupolds, we have reasonably tough scopes. the 6x36 is a solid performer, but comes in second place to the 6x42 - a much more fair comparison to the vx-3. if, we keep the discussion limited to the 6x36 and the 1.75-6, then the vx-3 wins because of comparable weight, better optics, and more versatility; the biggest factor being the superior optics. although the vx-3 is a variable, durability between a vx-3 and nearly any fixed-power is going to be close to a wash.
if we expand the discussion to include the 6x42, then it wins. it will have the best optics, the sight picture will always remain the same, and there is an advantage in durability (if mostly theoretical) w/ the 6x42.
it is my opinion that the leupold 6x42 is the best all-around big game hunting scope, bar none. optics are superb, it is simple, rugged, and light (something too many hunters overlook). put the 6x42 into a set of talley lightweights, and you have worry-free perfection on top of a rifle; the problem being that you must now find a rifle and stock system that is as solid as your scope and mounts. if you really want to go one more step, have a custom x-hair installed that already has the hashes in it for long range shooting, dialed in to your exact load (not an approximation ala the b&c reticles, or the bdc reticles). careful if you do that, though, because it really marries you into your particular bullet and powder. an alternative to that is to have turrets installed on a 6x42...