I went with the 4' because I mounted mine off the front bumper so it sits lower. If it was mounted higher I would get the 3'.

For trail use, either is more than sufficient. You're going to be limited by the weakest link in the group which is going to be someone with a basic handheld anyways (which I've found is still good for most trail rides, even when the group splits).

Either way, go with the tuneable tip.