In reply to Elsier:
I always go with a wheely duffle for checked baggage and my 30l climbing pack as hand luggage. Having a big rucksack and a smaller one is a nightmare. Putting the small one on your front looks stupid and you trip over things.
My wheely is a Blacks 50l buts it holds more like 65l I bought it in a rush and its lasted pretty well but it's starting to show signs of failure after about 20 long haul trips. 6-7 years worth. Patagonia do a nice looking one with proper replaceable wheels that have skateboard bearings. I'd consider that if you have a bit to spend. Importantly you need a nice long extending handle to make towing and not catching your heels comfortable.