With the same motors/props, and the same flight controller:
Quadrocopter - more lift (but also weights a bit more and draws more current), more props = more stable, less complicated (no yaw mechanism), but also slightly more expensive (additional motor, esc, propeller), less maneuverable. Requires 2 counterclockwise props.
Tricopter - less lift, not that stable, more complicated (yaw mechanism), slightly less expensive, more maneuverable. Can fly with all standard props.
If you can afford it - do what I did - buy the parts for tricopter and quadrocopter. Then it's just a matter of simple frame reconfiguration and reprogramming your flight controller... And if you decide that you like tricopter better - then you will always have a spare motor and ESC.