I've never looked through any Leupolds, but I do have some Nikon Monarch ATB's. I bought the 10x36 because they were a good bit smaller, but I ended up going with the 10x42's because that's what I really wanted. I actually don't have a reason for the decision other than I had the 42's in mind, but found a good deal on the 36's. I've tested them both and find them both equally clear and very, very good. The 36's just run out of light a little earlier than the 42's, but that's to be expected.
BTW...Shameless plug...I'm selling the 36's in the classifieds.