If you're purchasing a new BX series and you want a midmount mower I would go for the rck-54P (P for Pro I believe). It's the same deck used on the Kubota "Z" mowers. Not much price difference (100-200) but you could probably haggle with the dealer and get it for the same price as the cheaper rck-54.
I opted to out of the midmount when I purchased mine and got the Woods 54 finish mower for the same price. Easier to hook up and remove. I didn't want the deck hanging below the tractor due to some of the places I take it.
Hope this helps.