The Contour Crossbow 45 wouldn't be my own first choice. The shape narrows awkwardly at the top, it has a zip opening, and it's hard to carry axes and crampons securely on. I have an earlier version, which is great for skiing, as it holds the weight high, stable and close to the back. These are not essential qualities for the walker however, in fact they may make for a less comfortable carry.
For comfort and practicality, I would go for a classic top loading sack. Think about what you need to carry and make sure whatever sack you choose can carry it on the inside or outside. This is especially important if you carry crampons, as it's surprising how many smaller sacks don't have adequate storage for these. I use a Pod Black Ice, which is faultless but something of an investment at around £130. The ME Diamond 50 or LA Alpine Attack are good sacks at a lower price.
I would question though whether you really need a 45/50 litre sack. I only use one this large if I'm carrying both camping and climbing gear. You might be able to get away with a 25/30 litre for day walks, which would keep the price down and give you more choice.
