The 81' Invader also has the Vari-Ride skid and larger ski springs (this is why the 81 is my favorite). The LTD has more power, but it is also heavier with the larger radiator and twin pipes. And of course the looks...Although subjective, I much prefer the sleeker look of the Invader/Interceptor over the extra hump that the LTD's have (still beautiful sleds).
The 4/6 is definitely faster, but it has it's down sides as well. Parts can be very hard to find and that motor is prone to cold seizure. So be sure to thoroughly warm the engine up before riding it... I believe (if I remember correctly) the 4/6 has an older ignition system with an inferior timing curve.