Only speaking for myself, I won't presume to speak for any of the others here, but it depends on how you are turning pens... if I am on a mandrel and turn both pieces of the pen at the same time, I like a 6 or 8 inch tool rest. If I am turning on adjustable/between centers and only 1 barrel at a time, I like the 4" because I can get closer to the blank. A 3" would be better for between centers, IMHO, but I don't have a 3" and don't want to cut the 4" any shorter because I use it for other things.
As for better design, IMHO, that is a personal decision as to which you prefer for your own comfort. I broke the cast metal TR that came with my 1014 and replaced it with the 3 piece round bar system that PSI sold... after I got used to using it, I like it. I still have a classic design for my 1442, but I think it's too long and need a sorter version. I bought a series of SS round bars from Lee Valley or Veritas that fits, but I think the post is too long and I'm not sure how I could cut it shorter.. It's 1" SS stock and probably would take a while with a hack saw, plus I don't have a vice to hold it while I cut.