Zoom lens help

Hi,

I am new to photography and am looking for a zoom lens for my canon 10d. I came across a 135-400 4.5-5.6 and a 50-500 4-6.3 lens.

My question is this: The 50-500 seems more versitile with the larger f stop and zoom, but what is sacrificed for the broad range?

If I were to have to choose between the two above, what would you guys recommend?

Thanks,
Dan

There's more enthusiasm for the Canon 100-400mm f/4.5-5.6L IS USM than for the Sigma lenses you mentioned.