For me, and I did, I'd go the 23 spline and be done with it for a LONG time!!
V6 = for sure 23 spline
4 cyl = maybe 21 spline but I would still lean toward 23 spline just cause I like peace of mind
Make sure that Marlin has the 23 inputs in stock. I had to wait for months for them to come back in and they never did but I found a guy who had a new one for sale