In my opinion, the Swarovski Z3 3-9X36mm. is a better scope than the 2.5-8x36mm. Leupold (which has an actual magnification range of 2.6-7.8). I've owned both, and a Swarovski 3-9x36mm. has sat on several of my hunting rifles (actually mine were the 3-9X36mm. AVs, which are pretty much identical to the Z3s). There's nothing wrong with the Leupold, but the glass in the Swarovski is noticeably superior, providing a sharper, clearer, brighter image. The two scopes are very similar in external dimensions and weight (both 11-12 oz.), both being pleasingly compact and light, and having quite similar FOVs at the low end (actually the Swarovski has a slightly wider FOV at 3 power than the Leupold at 2.5). Swarovski also make a 3-10X42mm. (its actual magnification range is 3.3-10), but, for general hunting, I'd take the 3-9X36mm. because of its wider FOV at the low end and its more compact size, with an objective bell OD of 1.65" (vs. 1.89" for the 3-10X42 mm. and 1.60" for the Leupold). In my opinion, the Swarovski 3-9X36mm. comes very close to the perfect hunting-rifle scope unless very long shots are the norm.
Here's a review, from the Optics Talk forum, of the similar Swarovski 4-12X50mm. model. My guess is that what is said about the optical quality would apply equally to the 3-9X36mm.
http://www.opticstalk.com/swarovski-z3-412x50-brh-reticle_topic27412.html
One small difference between the two scopes is the distance at which they are parallax-free. With the Leupold, it's 150 yds.; with the Swarovski, 109 yds. (100 m.). Not that it makes any real difference in most practical hunting situations, but of the two, I'd prefer the 150-yd. setting, since it will result in slightly less parallax error at longer distances like 350-400 yds. Swarovski will change the parallax-free distance if this is desired (as will Leupold).
Only you can know whether springing for the Swarovski is financially justifiable given that you already have the Leupold 2.5-8X36mm., but in my opinion, there's no question as to which is the better scope.