Replacement for 18-55m kit lens

Hi,

I'm after a replacement lens for my 18-55mm kit lens. I've got a nikon d5000 and also a nikkor 70-300mm vr.

I'm after a walkabout lens to do most of my shooting, I'm about to embark on a trip around the world, so would like a lens that will do my trip justice.

Currently looking at the Sigma 17-70mm to go with my 70-300 or the other kit lens nikkor 18-105mm.....

I've read loads of contrasting reports about both now, and can say I'm pretty confused!! I'm by no means an expert, however want to do my trip justice when I look at my snaps.

So basically I'm after something to keep on my camera most of the time, to do the trick without changing to the 70-300mm

Any thoughts on which to get of the above, or any more?? My budget is approx £350

Cheers