I started with a cheap Kelty pack that did the job, went to a Mystery Ranch 6500 and really liked it, tried Kuiu (tried saving weight, not worth it), tried Kifaru (spent too much time on Rokslide reading reviews), and am now back with MR. A buddy just bought a Stone Glacier pack and that thing is solid as well. They all feel pretty good with 50 lbs or less, but for me the MR pack is what I like when I start packing heavy loads and meat. I'm probably in the minority on this forum, but I don't like the floppy Kifaru waist belt and softier, floppier bags. Unless the bag is full, I wound up spending more time than I should have messing with extra straps and buckles, getting everything tucked in place and tightened down. I was running a Tahr bag with a few extra pockets and guide lid. Other people really like that setup, so maybe it's just me? On my MR packs, both Guide Light and NICE frame, the pack bags, straps, and waist belt are stiffer, and I just like that better. My only dislike so far with the Guide Light frame is MR got rid of the pals webbing on the Guide Light waist belts. I've been thinking about sending mine off to pods8 (Rugged Stitching) to see if he can remedy that... maybe after this season.
Regarding which bag, I love the Pintler for 1-3 day hunts. I use this bag for pretty much everything. The 6500 (redesigned as the Marshall) is great, but it's a big bag. I like it and it does compress down, but just know that it's a BIG bag. There is a lot of room in that thing. I finally bought a Metcalf earlier this spring, and have had it out for a couple day hikes and shed hunting. I'll have that one out backpack hunting this year, and it should fit the bill nicely.
As has been stated above, it really is a great time to be shopping for a hunting backpack, or hunting gear in general. So many good options to try, and if you shop around or buy used you can resell and usually not lose much if any money if you take care of your gear.