Hi,
I have a 54 on a 4010 and cut maybe 1.5-2 acres (never measured) and have many obstacles to mow around plus slopes. Typically it takes me just over 1.5 hours to cut. Generally speaking, I'm happy with it and it does a great job.
However, there are times when I would really like to have the 60 inch deck because is reaches our farther from the tractor. For example, I have a stream running through where I cut and it's about a foot drop in most places from the edge to the water. I want to cut as close to the edge as I can yet keep the tractor away from the edge. So the extra several inches sticking out the side would be nice for this. I think it will also provide a nicer arc in the turns because I think you pick up most of the extra width on the left side, opposite the discharge. Not sure yet on this part.
Decks are hard to find used and I couldn't stomach the $2400 that the dealer wanted for a new deck (compared to $1800 I could have paid when I bought my tractor). I guess most people who buy a deck keep it. I've spent the last year hunting for a good used deck at a reasonable price and finally found one just the other day. I'm anxious to get it hooked up to see if the advantages are going to be as I hope.
If $ is tight, the 54 will do the job. If you can swing it, I'd say spend the extra up front and get the bigger deck. Also, the bigger deck heavier gauge steel and just an all together beefier piece of equipment.
Hope this helps,
Cheers