I've owned and still own both. For the use you describe, a light big game rifle or if you were going to use it on a walk around varminter I'd go with the 2.5-8x36. IMO the strong point of the 3.5-10x40 is versatility. It's a scope you might be more inclined to keep around to use on something else if you ever got rid of the rifle it was on. I like smaller scopes but the 3.5-10 is light for it's magnification and to me brighter with better eye relief than the 2.5-8. It'll work for small targets, long range shooting, varmints, big game and is easier to mount on long action rifles.
