3pk has most features. but you need specific servos depending on the receiver you have (digital or analogue).
m8 has less features, but still a top of line radio (was for the last 8 years anyway, now superseeded by the M11) .. can use any servo. i tried my friends m11, has lots more feature, but then took longer to change settings coz it just had so many functions. just couldnt change functions as quickly as my m8 on the fly.
3pj is 3pk's little brother.
i have m8 and find no need to upgrade to anything else. its easy to use and no complications with HRS mode and alike.