Serendipitously, I used a Mariposa+ on the PCT, swapped from that to a Circuit on the AT in 2010. I ordered the Circuit from one trail town, picked it up at another trail town and in the grass next to the post office I literally transferred everything from one pack to the other and then mailed the Mariposa+ home. Waist belt was getting too wonky to continue thru-hiking, but I figured maybe I could repair or use for day hikes or something.
Anyway, my recollection is that the capacity is pretty similar. I got everything from the one into/onto the other, at any rate. Maybe the big difference is just that with the Mariposa+ I was using a nightlite pad as pack padding, and the Circuit has that built in, but it was easy to strap a sit pad onto the top of the Circuit.
While heavier, I like the Circuit better. I love the waist belt pockets, and I like having the ability to adjust the pack strap connection to the pack itself --- at times my Mariposa+ was sitting with a significant gap between my upper back and the pack. I had sort of gotten used to it, but my (ULA carrying) hiking partner gave me a lot of crap about how stupid it looked and that I couldn't possibly be comfortable.
I like the fact that the side mesh pockets on the Mariposa Plus are deeper, but I've adapted my external packing style to work well enough with the circuit.
One key thing I like about the circuit is that it just lasts longer. It really does seem to me that you get about one thru-hike out of a Mariposa+. I did the latter half of the AT and this past year the CDT with my Circuit and it's still good to go for another long trip. So I guess it's a combination of comfort, a limited set of features (just the ones that are actually worth the weight), and durability that make the Circuit worth the extra weight. Ah, I might also add that I had the Circuit loaded pretty fully going in to the snowy start of my CDT trip last year and it rode/carried definitely better than the Mariposa+ did when I started into the Sierras similarly overloaded on the PCT.