It looks a bit cheap for a backpack of this size. When buying one you really want to get something of a decent quality, a good backpack will last you for many years, probably decades. Having a detachable daypack seems handy, but the way this is attached to the backpack is the worst possible, at least if you actually have stuff in there. When carrying a large pack you want to distribute most of the weight close to your body and/or low (unless you do technical climbing, then you want the weight on your shoulders), the daypack hangs high up far away from your body.
Apart from that I can't say much except for that fit really is important and you should go for what feels right.