I'm constantly struggling with this one, but I'm a gear nut:) Last season I skied a few different multi-day packs. I'm a total newbie, so please take this with a grain of salt....
Arctyerx Arrakis 50
I fell in love with this pack right off the bat, but it's not perfect. The bad first. It's heavy at five pounds. Unless you lock down the hip belt sway straps, the belt pops off the pack at the most inopportune times. There are no compressions straps to shrink the pack down. Lastly, although it has a number of exterior attachment points I don't believe they are as flexible as compression straps.
The good is pretty good though. It's the most weather worthy pack I've ever owned. The snow tools kangroo pocket is a great size. More than enough for an avy shovel and even a few other bits of gear. I've been known to stuff a wet tent in there and lash my shovel to the outside. The interior of the pack feels well lit because of the whitish coloring which means finding gear is a lot easier. The side access zipper is a bit awkward, but definitely serves it's purpose. I really like the suspension for winter time travel. It doesn't inhibit the breathability of my jacket back panel, and it moves very well. The pack is a pretty good load hauler as well. I'd say max out is around 50 lbs. It's a firm, resilient pack. I use Voile ski straps instead of the nylon for the exterior lashings. It does not like the thicker BD ski straps, but they can be made to work. The lash points make for a huge variety of options. I also love how convenient the roll top is. Press down, roll, clip done. I have no issues leaving this guy outside in bad weather when making camp. I also like that the aluminum stay can be removed for use as a splint or any other make shift need.
Be aware that you have to find ways of stabilizing the gear inside if it's loose. The lack of compression makes skiing this guy interesting if your load is moving around inside.
Dakine Guide 50
It looks like Dakine won't be offering this pack this year in 50 litre, but is still available in the smaller "poacher". I really liked this pack at first. The Bad: The shoulder straps are pretty anemic for my taste. They need a little beef to hump 50L. The insulated hose routing on the shoulder strap worked pretty good, but was pretty tight. The location of the bladder pocket however, on the out board of the interior, was not cool. It placed a good deal of weight out away from my torso leveraging the suspension. Not a fan. The pocket was also lacking a hook for to attach the bladder to. I ended up keeping my bladder against the back zip panel and just moved or pulled it out when need access to the main compartment. The back panel is very well padded but soaks in moisture like sponge. It froze up on me during a number of overnighters where it was sucking the perspiration moving through my layers during the day. I also had an incident where the aluminum stays punched through the pack floor and speared me a bit while on the move. Bummer. Lastly this guy is not a heavy load hauler. Max I'd load is about 32-35 lbs total. The shovel/probe pocket were a slight bit small but definitely doable as were the Nalgene pockets. Just be sure to pack those guys first, gear in the main compartment presses these pockets out and makes for unhappy moments. This pack does suffer from the anemic strap syndrome that is sweeping the outdoor industry as a whole in an attempt to save weight. But in this case not overly so.
The Good: I really liked all the features. Everything from the innovative diagonal ski carry to the helmet carrier were pretty cool. The buckles are very glove/snow friendly and the waist belt has a huge amount size range to accommodate layers. The primary Ice Ax carry system is very cool unless your using bent shaft tools. All the pockets were cool for organizing smaller items and maps. It carries pretty well and skis about the same. The material was my favorite being a strong ballistic nylon that proved to be very abrasion resistant. Except in the areas where the less sturdy grayish nylon is used. This seems to tearout along seams.
My climbing partner has been using a Gregory Alpinisto 50 this last season with great success. It's stable and feature rich. I don't like a lack of avy tool pocket, but other than that love the pack. It has the normal Gregory robustness. I just don't think it's a great Ski pack, but it can serve that purpose.
On a quick note, I'm not overwhelmed with the REI snow sports lineup. I'm not sure what's going on in this industry, but it feels like the straps get smaller every year and the colors are really going sideways.
Right now I'm looking at the Dueter Tour Guide 45, the Osprey Variant 52, and The Mammut Guides as well. Just like stove and tent selection, this is a tough one Hope that helps out a bit.