I understand from the other posts that you are going on a 5-day trek. You will experience temps between 0 and 25 C. You will have to hump your sleepingbag, a mat, a tent (or your share of the tent because you and your partner can split up the weight and bulk), and your personal gear. You want to keep the load below 10 Kg. It has already been suggested that you rent or borrow. I'd offer the same advice. Choosing a pack is easy compared to picking the right boots, because if you are wrong the consequences are not nearly as bad. Carrying an ill fitting pack is no fun, but wearing the wrong boots or new boots is torture. There are all kinds of packs out there that are high quality and some junk too. I'll discuss features and let you research a particular brand's quality. The different features that make one pack better than another is largely a matter of preference. When you buy, rent, or borrow you will be bombarded with all sorts of choices that you will have to make. Having said that these are my preferences.

TOP LOADING. I like being able to shove stuff into a single bag and tighten the lid onto my load. Other people prefer a front, or panel, loading pack because they can organize their gear better.

A GOOD SUSPENSION SYSTEM. I like my weight to ride on my hips. I have a buddy who hurt himself in a car wreck; he likes his weight on his shoulders.

EXTEROIR POCKETS. I look for one in the lid and one on each side. This way I can separate my rain gear and fuel bottles away from the rest of my load. I want to do this because I want to get to my raingear without having to open my pack and fuel bottles make everything in my pack smell like fuel. I also hang a water bottle holster and accessory pocket off my hip belt for water and little things like lip balm, a compass, and T.P.

SIDE COMPRESSION STRAPS. I like these because they keep the load tight and I can slip my sleeping pad in on one side and a large knife or tent poles in on the other.

I wouldn't get too worked up about the pack you pick, because as you become more experienced your needs will change. Just find one that will work for this trip, learn from it, and have fun.

-Biscuits