RE: Boy, do I feel stupid!
Personally, I would get the 50-cal 1-32" twist -- then the 54-cal in 1-66" twist.
I am not a proponent of using a 45-cal sabot/bullet in a 54-cal rifle. To me, that seems like a completer waste of what the barrel is designed for -- that being a much larger hole.
So unless you intend on using conicals in that 54-cal fast twist barrel, I would not buy the 54-calfor shooting smaller sabots.