I would guess that the 3PM is lighter than the PKS, but I've not held a PKS in a while, so really not sure...
You could go to Futaba's site and compare weights...
My 3PM is light. I don't even notice it in a 30 minute main, so I'd say that you were safe with either.
I've held 3PKS systems in the past and they were really light as well. I didn't get a chance to hold them both back-to-back for comparison purposes, but I'd bet that the 3PM system is lighter... It is really well balanced. So much so that in over 2 years I've not given it any thought at all in regards to its weight and balance!