17-85mm VS Sigma 17-70mm

Hi,

I'm about to pick up a Canon 40D and lens, mainly for travel/landscape photography (with a plan to take it underwater soon). I was looking at getting it with the Canon 17-85mm lens, but have read some reviews that the Sigma 17-70mm isn't too bad for underwater.

So, for an above water lens, how would these two compare? Any pros and cons on each that anyone would like to list? Which would you pick for an a topside lens?

Thank for the help,
Ryan.