I have been trying to decide this same thing Keith! I talked with AWOL about buying a new 543 and have him port it right away. He said for the cost they weren't worth it. He suggested the 545, but for less then $100 you can get more power at the same weight with as a 550.
As for the 421. When I looked up specs on it. I believe that they are around the same weight as a 346/550 but iirc a smaller motor. If I'm going to carry around the weight. I am going to have the most power I can.
I have a ported 562 and I also run a 20" but a sugihara. I love the saw and it can be used for limbing, but like you already have said it is too tip heavy. It's nice though to be able to start from the top and walk up most of the tree without switching saw's.