Comparing manual trans options - variables other than gear ratios?

My '69 Dart had the large retainer, beat the crap out of that car. Never had any issues with the large retainer, don't know if I understand what you mean. What I do know is, it was easy to replace the 2nd gear blocker ring that I broke a lot from power shifting, because I had the stock shifter. The small retainer is a pain for the same procedure, you have to pull the tail shaft housing and all to replace that same ring.
Good luck with your end choice, be happy with it!