The thick and rich texture of oil paints lends itself to a more sculptural effect. Using brushes and knives allows me to buildup the textures on the canvas. Also oils can be worked on repeatedly over a long time period, thus allowing for multiple edits of the work.
Watercolors have such an immediacy and translucent quality that I particularly like to use this medium to convey water and sky. I have used pastels, silk screen, and block print making but I always return to my brushes and knives.