Hi I have a chance to get a keep all but I really won't I don't know if I'm going to get the 50 or the 55 it would be for a carry-on on the airplane or just for a weekend bag. I definitely want to make sure that I got the right size I have the chance to get a pre-loved keep all 50 with straps and everything for $800. is it worth it or should I just buy a new one please give me advice! Xx