buy a double-lined one.
(berghaus do goo ones, I think it's called drysac or something. that way if your shoes get covered in shit, you can put them in the bag but not inside the liner. same with wet clothes. for a hot country, serious mesh venting on the back where the straps are is important so you don't end up sweating line a paedophile in mothercare when you're carrying it.

Otherwise, try them, see which feels comfortable. You're probably going to see little change from £150 for a good one, so take your time and be picky.
( the mighty badger Aphrodite, on a bar stool, by your side, Mon 1 Jun 2009, 14:06, archived)

I'm trying to get away with spending #100
Hmm...
www.b3ta.com/talk/6189837
(rnuk http://flickr.com/rnuk, Mon 1 Jun 2009, 14:07, archived)