The Quantum Competition 3, is small, has lipo voltage cutoff ability, nice if your planning on the lipo battery route, also has solder tabs for the power wires. Has different profiles for tuning. Newer, most recommended, and will have better future factory support.
The Quantum Competition 2 , is small and has solder tabs. Also has different profiles for tuning.
The v7.1 is extremly reliable, and heavy duty, very easy to setup and use. Although it dosen't have solder tabs, so easy wire replacement is out, and its larger than the previous two. Has plug-in current limiter chips that have varible throttle curves.
All will work well and can win any race. Apply a decal around the seam of the case to aid keeping the case together.