I've been very happy with the cut quality of my
Bx2680 with the 60" deck, I also had a
bx1870 with 54in deck for a short period of time (it was bought to be sold). Both were excellent mowers.
I would say you won't notice a lot of difference with the 54", in fact it seems to mow a little bit better in certain conditions.
I've had a number of comercial mowers, Hustler Z, Kubota Zd326 and 331, wheel horse 520 with comercial 60" deck, and coming from those I was worried the BX cut quality would be a disappointment but was pleasantly surprised. I would put the cut quality right up there with the ZD's
I don't have much experience with the larger JD garden tractors, but for the money, I can't see why to go with one over a BX.
Also, remember that cut quality for different manufacturers will vary on grass type. My Hustler with XR7 deck was designed more for Bahia grass and struggled with lifting fescue during the summer, which tall fescue is what I have.