I prefer sinktips for two reasons:
1. Increased visibility and ability to judge the direction of the line especially in moving water. I have a really tough time with this with full sinking lines. Dark line going straight down right off the tip of the rod makes it hard to judge what the fly is doing. I've found that the increased visibility of the line greatly increases my ability to fish it effectively.
2. More snags with full sinking lines. If your fishing the bottom of the water column I've found that full sinking lines run along the bottom dragging the fly through rocks and other structure. With tips you get a downward angle from rod tip to fly but the floating section keeps it off the bottom reducing the number of hang ups. I also have found that you get more of a belly with sinking lines because the whole line wants to sink but the rod is pulling up creating the belly. Sink tips give you more of a straight line diagnol from fly to rod increasing ability to feel strikes.
Hope that helps,