I prefer 3 under, it puts the point closer to the target for me. When I made the change from split to 3 under I quit tearing up my fence. I think that I shot split finger with a compound for so long it just never seemed natural to me until I made the change, even then it took some getting used to but the rewards have been plentiful. I personally believe that 3 under is the ticket, if for no other reason the guys that I've seen win 3D shoots all shoot 3 under.
I set most of my nock points on the string at around 5/8 inch above the shelf for 3 under making it a little higher than split finger. I was lucky to get my first custom bow that was made for me with a larger grip for my big paws and it is tillered for 3 under. The porpoise that you described is most likely from your string nock placement.