Upgrade from kit lens for Travel?

Hi,

I'm looking for a new lens and filter for my Canon 550D/T2i.

I curently have the following:

• Canon 18-55 IS Kit Lens

• Canon 55-250 IS Zoom Lens

• Canon 50mm 1.8 Lens

The 50mm lens is great for portraits etc and I really like the quality of it, however whenever I look at the pictures from it and compare back to the kit lens it just leaves me wanting a sharper lens for my main walkabout lens.

The 55-250 IS lens I am happy with as it is not that often that I have to zoom and I find the quality of it is very good. I know it should be the same standard as the 18-55 kit lens but I feel it is a touch better.

I am heading to South America travelling for 4-5 months so I was wanting to get a new as sharp as possible lens to remain on the Camera the majority of the time for travel photography. Landscapes, people, everything really...

I had a look at this Sigma 17-70 OS lens: http://www.amazon.co.uk/dp/B002ZNJB32/

It is around the price range I think I can afford, although if something exceptionally good was available at a touch more I may consider it. I feel the Sigma seems to have a good walkabout focal length with a little extra reach than the canon kit lens.

Any help is greatly appreciated.

Thanks in advance.