I went with the 54". I have only about 1/2 acre lawn now, and working my way out to about 1 1/2 acres. If I could do it over again, I would spend the $500 extra and get the 7-iron. It is a much tougher looking deck.
I tried both, besides a bit of speedup the main differences are ease in edging and inner radius of cut. Having the edge be further out is nice in that you are a couple more inches away from that pesky low branch (ease in edging). The inner cut radius thing is the nicest part. Since you are a couple more inches out you can turn less sharply to go around a given tree, this means less lawn damage and a generally easier time. I hope that explanation makes sense.
My suggestion before PLUNKING down any thing is go to the JD site and look at both. Then visit your local dealers, talk to a salesman and obtain a brochure. I think you will find a large amount of difference between the 54 and 60 inch deck for the 1026R.
The 60d deck is a 7 iron deck meaning the housing is made from 7 gauge steel. The deck height above the rotating blades is higher meaning more air space which results in better air flow and a better mulching cut if you use the mulches kit. Besides that, the 60d costs more! I have the 60d as it was installed in error at no extra charge but the mulching kit for it cost me more than the 54 deck. I mulch and always have. The 60d is amazing for mulching.
Another bummer about the 54D auto-connect for the all new one series is the "ventilation" of the top of the deck. There are no less than twenty holes of various shapes and sizes (round and square 6mm - 10mm). These hold all kinds of clippings and create places for the deck to clog. So PLEASE!!! get the 60D auto-connect "7-iron".