I've gone through several bags in my lifetime, but this one is definitely the most uncomfortable. It's too bad - I was stoked to receive this. The big lower back "pad/knob" that is the centerpiece of their "ergoactiv" system is horribly uncomfortable. It just digs into your lower back, and rubs around constantly causing a lot of discomfort, soreness and chaffing. After about 30 mins of hiking it just feels like a hard knot. The shoulder straps are also very thin and barely padded. Honestly, it's a horrible design - one that I believe is providing a solution to a problem that never existed. Traditional non waist rotating backpacks are WAY more comfortable than this. I've put 30lb in this pack and taken it a total of 18 miles since I got it, and I wouldn't go another mile with this thing. In fact, I just bought (for $99 on sale locally) a Deuter Guide Lite 32+ to replace this and it is incredibly comfortable. Deuter knows comfort. Black Diamond could learn a thing or two from them. I'm sending this back this week. Do yourself a favor and avoid this bag and go with Deuter - they are unbeatable in terms of function and comfort.
This thing is sturdy, easy to open, easy to fill, and easy to drink from. It can also be tossed around and dropped and take some serious abuse. I also like the ability to clip it to your bag with a biner, and save room INSIDE your pack for other necessities (not that water isn't one) because of the plastic lid strap. I love this thing! Probably won't be my last.
I got the 38" one for my 06' Impreza (per Thule's recommendation/configuration tool). It's a little short on both ends and leaves about a 1-2" gap on the driver/passenger side but it has no bearing on it's effectiveness. I just thought it would reach to the ends so it's flush with the crossbar mounts. That said, it works perfectly and cuts down on the wind noise from the crossbars. Assembly and installation was fairly easy, but it does feel kinda flimsy overall. Nevertheless, once it's installed you don't need to ever touch it and it really reduces/eliminates wind noise.
I just bought this in medium and large. I'm 6'2" and 175 lbs. The large was way too baggy for me in the waist and mid section, even with a fleece or light sweater on beneath it. The sleeves and length in the arms however was just right if not on the slightly longer side, which would be perfect for you. The medium fits me really well. The arms do ride up very slightly compared to the large, but this is a non issue unless you constantly walk like a zombie, with your arms out.
I ended up sending the large back and keeping the medium. I love it! I think you'd be best off going with large.
As for the other brands, I can't speak for Montbell but Marmot clothing trends to run smaller/tighter than others. If you're not sure, order multiple sizes and just send back what doesn't fit :)