Depends: are you sketching (with pencils, presumably), or are you painting with watercolors? :)
If you're sketching/drawing with pencils and other dry media, the sketching paper is probably to be preferred, because the vast majority of what is available as "watercolor paper" is cold-press, i.e. pebbly/rough surfaced.
If you're painting with watercolors, unless you're using a particularly dry technique, the sketch paper will probably simply fall apart on you. You need to use a paper that is designed to withstand watercolor techniques such as washes and blending, which pretty much means using the heaviest-weight watercolor paper you can afford, or using a lighter weight and investing in the equipment to stretch it as it dries. Either way, though, a paper that tears when wet will not cut it.
And if you're doing neither sketching nor watercolors, but, say, pen and ink drawings, neither sketch paper nor watercolor paper will be particularly satisfactory. Moral of the story: match the paper to the material/technique.