elex300 pretty much gave it in a nutshell, but I will repeat with my two cents.
4-pole motors have a smooth powerband with high torque; they can pull heavy vehicles better, but may not be ideal for lightweight setups unless you have lots of room for gearing changes.
2-pole motors tend to rev higher and make lots of power at higher RPMs, but may limit your gearing if you have a heavy car.
In my opinion it doesnt make much difference for most cars. But certain vehicles are heavy or under a heavy load and only have so much room for a bigger motor. In this case a comparable 4-pole motor would run smoother, it would run cooler and it would increase gearing options.