I have not gotten out to test mine hiking yet, but I found the fits very similar between the Mariposa Plus and the Ohm as far as how the load carries. I love everything about the MP, except that the straps on the MP were very wide (measured on a medium they are 4 inches wide).
When I loaded up the pack and put it on, they stuck out like little wings, probably only made worse by my female figure, and then also bunched up above my armpits. When walking with trekking poles, these "wings" stuck out far enough to rub on my inner arms. I hoped using the pads in the straps would eliminate this problem, and while it helped a bit, the wings were still there, and rubbing.
My other issue with the harness on the MP was that the sternum strap was much lower than on any other pack I have, and went right across my business, again probably making the 4-inch wide straps stick out even more. Perhaps a guy wouldn't have these issues, but I really think the straps are excessively wide! I understand that it may be possible to have a modification done by the manufacturer on them, but I ordered an Ohm for comparison.
The Ohm is very comfortable (so far I only loaded it to about 20 pounds) and I feel like for a similar weight it is a bit more fully featured than the MP. I like that it comes with several accessories and that I can configure the pack as I'd like for each trip. For me, I like the hipbelt pockets, and the lack of a beefy belt doesn't bother me, I rely on it for stabilization more than weight transfer. Oh, and the internal pad straps are nice! The shoulder straps are somewhat narrower than on the MP, and the sternum strap is slightly higher (though still below what I'm used to?). I am also a fan of load lifters, as they help me reposition the weight during the day. So I think I'm sticking with the Ohm.
Overall I think the packs are very comparable, and the biggest difference is how the pack fits your body (well, isn't it always). I'm sure the wider straps on the MP fit lots of people well, just not me. Both manufacturers have good return policies so you might want to buy both and see what you like!
Oh, I just wanted to add that both packs are the 2010 models, and I believe I have the correct sizing as I tried my roommate's 2009 MP in small and it was definitely too small....