I have both, snow shoes and two types of touring ski. Depends on where and how far you want to travel in the backcountry. Terrain can be steep and not very much fun for skiing if you aren't a hotshot expert with a heavy pack on your back, e.g. Mountaineer's route up to Whitney.
I could ski segments of the Mountaineer's Route, but for me it isn't really worth bringing the ski gear to that place. I used snow shoes one year (big winter, tons of snow down to the lower parking lots, and approach plus up then North Fork was made a lot easier with snow shoes compared to postholing on foot. Ski may work in some places, but for much of that ascent we just walked on boots and higher up with crampons. Late in the day heading down things got real soft, which is when snow shoes help, but I still poked through the surface at times.
The main trail - again, on and off not really a place to ski unless there's a ton of snow.
Anywhere else, deep into the Sierra, long pass approaches and slow descents and ski are the way to go. Especially ski you don't have to constantly convert with skins (e.g. Voile touring ski with partial fish scale bases), which may not be fast and fun when you head up there for the skiing itself, but they work great to cover ground.
I have a Marmot 0 degree bag, I think, but have also used a Mountain Hardwear Wraith -20F bag. Now that one was far too warm even at 12,800 feet in a 50mph windstorm wearing nothing but base layers in the bag. It was bulky and difficult to pack, so I got the Marmot which was fine the next time I went up in slightly better conditions.
Packs - I use big packs and in winter they are barely large enough. My Gregory Whitney 95 is fine, but when the trip is longer, I may use the Gregory Denali 105 liter pack. My entire gear philosophy is "it cannot fail" which more or less rules out anything lightweight when it comes to critical items like boots or pack.
Winter packs need to be able to haul heavy loads in winter and you need to be able to fit much more stuff, foot, fuel, etc. I couldn't bring a 70 liter in summer, so your mileage may vary. I carry tons of photo gear and these winter tents, down filled sleeping pads, insulated layers of clothing, larger snow melt pots, etc.
If you have a 70 liter pack just try to pack a 4 season tent and a winter bag into it. You may be just about full at that point. strap a snow shovel, crampons and ice axe to it, a stove, fuel, food, a few layers of clothing, and you get the picture.
This image shows you what a smaller pack looks like in winter:
Winter trips are really best done in a group, not just from a safety point of view, but very much for gear sharing, as the gear is heavy. A shared 8 pound tent is always better than carrying a 5.5 pound tent by yourself.
