How are you intending to travel and move around while you are in situ? If you will mainly be staying in places for relatively extended periods of time but taking a fair bit of kit then a more solid suitcase style of bag might be in order as they are easier to pack and live out of, wheels rapidly become useful if you will be moving about relatively flat areas for distances. The Da Kine Split Roller has a large load capacity (~100l) but is still pretty easy to heft around, has wheels and you can if you want just take the upper or lower segment for around half the capacity.
However it sounds like you might not be doing that since you have a fair number of countries with a relatively short space of time there. In which case a rucksack would be a better option, £70 might be a little bit on the cheap side for a really good quality bag but look for something with good shoulder straps and lumbar support with waist strap as well as some form of frame (internal is probably better here) as it helps it to keep its shape and will also mean that the bag isn't resting quite so heavily on your back. Bags with plenty of zips are handy for access but that does also work the same way for thieves so you could either get locks on all zips or use a liner for the rucksack and lock the most important zips. I'm not too sure what you should be looking at in your budget but Berghaus, Hagloffs (my choice and seems capable of withstanding anything short of a small scale nuclear war), Karrimor and The North Face all do options that are worth investigating.
Duffel bags I'm not quite so keen on, this is mainly personal choice but they aren't as easy to move with as a rucksack nor as organised as a suitcase plus they don't have wheels. They are however cheaper than both since they are simpler and they are probably easier to pack with a 'cram-it-in' style.
