Do you already have all the rest of your gear -- or at least the major pieces (tent, bag, pad, etc.)?
Your choice of backpack depends a lot on what you expect it to carry. Since you want to pack small and light, you should choose all the rest of your gear carefully -- especially your choice of tent and sleeping bag. You can then shop for an appropriately small and light weight backpack.
When choosing among packs that will carry your gear, fit and comfort should be the most important criteria -- then followed by price/features.