I have a very nice Nikon D700, Nikon 17-35, 24-120 50mm 1.8 and 70-300 lenses. I'm about to retire and have trips to Italy and Vietnam planned. As you'll appreciate taking all of these lenses and an - about to be added - couple of Lee filters it's quite a heavy load - the D700 itself is, albeit reassuringly, hefty. Just back from Barcelona, where I took most of this kit in a Kata 3N1-20 bag, bought when I had my D70 and a couple of lenses, I found the bag quite uncomfortable to carry round all day and so my dilemma is whether to get a slightly bigger but more comfortable bag or get an all-purpose lighter camera, say the Olympus Pen and smaller bag which would seem to kind of defeat the object of finally owning such a lovely bit of kit like the D700. Sorry to be quite so long-winded on my first ever post here, but suggestions very welcome.
Well, I also have a lot of D700 kit, though I used to have a nice compact Pentax K20 and a few lovely lenses. When travelling abroad the D700 adds the extra low light dimension that I often used to miss out on when exploring cities in the twilight or after dark, or in interiors where you cannot use tripods or flash. There is no way I would compromise on a smaller sensor nor, having made the investment, leave it all at home. At any ISO over 400 the difference between a D700 and a micro 4/3 is literally night and day.
However, I would concur that the design of ones backpack makes a huge difference to how comfortable you are when hauling gear around. Specifically, the width and amount of padding on the straps, as well as the shape of the back padding and centre of gravity, all make a huge difference.
Judging from my kit list I would say your load is "moderate" compared to mine, but compared to an APSC camera and couple of lenses it will require a better carrying solution. I tried the Kata you have, but for heavier loads found it very poor in the comfort department because the shoulder straps are way too thin, although it was a very clever bag. Also, when I upgraded to the D700 from a Pentax K20, I found even the few extra KG involved turned my Lowepro mini-trekker from trusted companion to pain in the neck - this time because of lack of padding and poor shape.
Over the years I have bought and sold a lot of rucksacks and very few have managed to combine the qualities I need in a sensible package, but the most comfortable I ever found for moderate loads (5kg) are either of these two.
If you can bear to leave one lens at home (I seldom use a long lens when travelling) then you could easily get away with this one:
http://www.warehouseexpress.com/buy...ler-Cupcake-Half-Photo-Black-Backpack_1524132
Plenty of room for D700 plus 24-120 in the lower section, plus a couple of extra lenses. Weight carried nice and low too. This leaves loads of room on the top half for the kind of stuff you normally need to carry on holiday (hat, sunglasses, extra jumper, maps etc.). I have walked miles (12 in one non-stop session the other day) with this pack, D700 plus 24-120 and 16-35, flash, spare batteries and a 12" laptop.
The full photo version would hold all of your gear very easily and would probably still be comfortable though I can't find any going cheap.
Both worth a look I would say, though don't buy one in a Crumpler dealer for £119!