i looked at both the kata 3n1 and the sling shot 300. i carry a canon 40D, 17-55 f/2.8 IS and a 70-200 f/2.8L IS both with lens hoods and speedlite flash with me. both were "ok".
i ended up getting the fastpack 350. a bit big, but i can carry my lap lop with me easily if i need it. also for longer day trips, i can put lunch in there as tons of other shit. for shorter walks, the side opens like the slingshot, you just have to take out your right arm. at first i thought it would be a PITA. after 2 weeks in thailand, i realized it was no problem at all.
heres a link to it.
fastpack 350/