Read about the pros and cons of the pocket sill vs slop sill replacement window.
What type of replacement window is better: a pocket sill or a slope sill? Pocket sill windows are a deep sill (frame) design that completely surrounds the bottom edge of the sash in the closed position. The advantage of this is in many cases you will get better air infiltration and more glass to your window. The biggest disadvantage is the sill looks and acts like a gutter for your window.
Slope window sills slope at the sill, flushing dirt and debris when it rains. This design uses gravity to send water out of the sill of the window. There is little to no maintenance with this type of window sill and therefore less of a chance of structural erosion of your window frame over time. Furthermore, slope sills often have the same if not better air infiltration rating and as much glass as the pocket sill. For these reasons, WoW generally prefer a sloped sill window. Read more here.