The pitfall that doesn't show up in a stats comparison is that the Sidewinder has a very good canopy view in the "Up" direction, which is important if you're combat-oriented, and especially if you use a head tracker like TrackIR or the Oculus Rift. The way combat is designed in this game, with roll prioritized over yaw, you're going to be "pulling" in the Up direction (with thrusters, if you know what you're doing) to get guns on target.
The Adder, by comparison, has a narrow slot of a canopy view and no "Up" view.
You can still use the sensor display in combat, but for those of us raised on air combat sims with (early) padlock views or (later) TrackIR, it's a very limited view for combat. Not that the Adder is a dedicated combat ship by any means, but since it has more self-defense than a Hauler, it might be something to think about.