I'm a big fan of my Pod Alpine 50. It used to be my "do everything" rucksack. I used it for days out, cragging, scottish winter, alpine, multiday hikes, I took it up Mera Peak, and a friend borrowed it to hike to Annapurna base camp. It has one of those folding closures that allows it to extend hugely (at the cost of being a little unstable when _really_ full), but also cinches down to be quite small.
The biggest issue I had with it was that I was always tempted to put too much in it! So, recently I bought a Mountain hardware scrambler 30 to force me to carry less!
The new Black Diamond packs are AWESOME. I have a Speed 30 and it's sweet - great suspension but strippable, not too heavy anyway and a nice simple design with everything you need and nothing you don't. Excellent ice tool attachments too.
They run larger than their official sizes I think, but I'm sure the 50 will compact well if you really need the size. The 30 should be more than enough though.
I have an Osprey Mutant and wouldn't recommend it. The back system has no support in it what so ever. The best thing I actually ever did to that ruck sack was put a sam splint down the back of it. I have used the Law Alpine Peak Attack (like the Alpine attack but smaller) and it was brilliant.