If this is for hunting and being put on an AR, then I'd lean towards the Mk 5, mainly because of weight/size. The glass clarity is plenty good for hunting and/or PRS/LR shooting.
As to which is better, that's an unfair comparison IMO, since they really sit in different price/performance classes. The Rzr GII has better glass hands down. Period. If you can't see the difference I would sincerely recommend either getting your eyes checked, or spend more time adjusting the diopter setting on the scopes.
As far as ruggedness, that's really hard to quantify without some 801G Shock & vibe testing. What can be said is that anecdotally, Rzr GII's have been shown to be incredibly robust and tolerant of a lot of abuse. The Mk 5's just haven't been out there that long, so time will tell (to be fair, I have had zero issues with the Mk5's robustness, and I tend to be a little rough on my gear at times).
So, which is better? Well, the Rzr GII has the better record for robustness and glass quality. Which is better for you, since you're installing it on a AR? Probably the Mk 5. Doesn't mean the Rzr is a lesser scope, only that for the intended platform, and your intended purposes (which is quite a spread) the Mk 5 may be a safer option (if you can live without illumination). If illumination is an absolute must, and you can tolerate a top heavy AR, then the 3-18 Rzr would be a solid choice.
For the record, I have both the 3-18 and the 4.5x27 Rzr, as well as a 5-25 Mk-5, and have spent considerable time behind all of them, in both match and hunting situations.