My take... The MR is way more durable, the Kifaru is lighter. Take your pick...
I have a MR 6500 and CC, as well as Bikini/Highcamp 7000 set up. The jury is still out with me. I've used the MR for about 8-9 years, usually on multiple 10-14 day trips each year, as well as many day trips. I've only used the Kifaru on two hunts, a 10 day sheep hunt and a quick caribou trip.
My only complaint about the 6500 is the weight. Fits me fine, and can load it up to pack more weight than I ever want to... I've packed tons of meat in it, literally... The pack can handle way more than my knees can.
The main selling point for me with the HC/Bikini over the 6500 is weight. I'm always looking at ways to cut weight. Currently I'm about out of options... my wallet and wife are both happy about that.

A side bonus is the j-hook set up. Pretty sweet to have basically unlimited lash points.
I like that the Kifaru is about 4lbs lighter, but so far the durability has been iffy. The belt 'power pull' doesn't work, the belt strap slips sideways when you pull. Sure I can work with it, but for $$$ it should work and Kifaru offered to make it right.
I broke one of the plastic slider buckles on about the second day of real use. One of the shoulder straps has a small tear in it at a seam. Uh what else...
You can't chinch down the pack like you can the 6500, not even close... I was to the point that I was afraid to pull any harder, as I think the webbing would rip off the pack where it's sewn on the body, or at a minimum break another plastic buckle. I'd pull the strap with one hand and cinch it up with the other... can't just horse on the tag end and get it tight like the 6500.
The roll top design doesn't stay closed unless you have a bit of gear in it, otherwise it comes unwound while hiking. Not a big deal, but after crawling through alders and spruce trees, its not cool to find a pack full of leaves and needles.
Better/easier access for a spotter would be a bonus. I didn't think this would bother me that much, but having to dig it out all the time is a serious PITA, and even worse when the pack is half full. I really like that the 6500 has a pocket on the front so I can stuff my spotter/tripod into for easy access. Didn't really like the option of strapping it to the outside of the pack, but I'm sure that would work too.
The lack of a divider between the sleeping bag area and the main section took a bit to get used to, but it worked ok. Having an extra set of chinch straps was a must for me. The lower chinch has to be sucked up to keep heavy stuff from migrating to the bottom of the pack, and if you want to chinch it up you need that center strap.
IMO the Highcamp could benefit from a compression strap across the top, similar to the 6500. I use it quite often, and found myself wishing it was in the HC bag. Sure is nice to have an extra lash point across the top, or to pull the top portion of the bag up to the frame when loaded.
The shoulder strap fit is great on the Bikini, and the pack is very comfortable. Not sure its really any more comfortable than the 6500 though. I ran them both alternating days all summer long with a 60lb bag of sand. No clear winner for me.