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If you are cutting grass like matter which is 18" or less in height all of the time, then you can go to the 60" mower. Problem is, while that may be the plan, you just might find yourself cutting a field which is much taller and with more dense and tougher cutting.
The real issue becomes how much material volume is under the cutter at any time and how effectively it can discharge the material after cutting it, not just knocking it down. The taller the material, the more dense the material, the more power required to cut it properly. You can adjust the tractor speed to accommodate these issues, but I have seen some who opt for a wider mower end up crawling along to get a decent cut when the cutting is on more stubborn weeds and less like grass.
When you run any properly sized and operating cutter over any material, you shouldn't be able to get much, if any material to stand up by taking your foot and rubbing it against the cutting direction of the mower after that cutting pass. If you find that much of the material in the field is just getting knocked over and not properly cut, it could be a number of things from the tractor speed too fast, the mower blade is dull, the engine RPM's aren't high enough to reach the proper PTO speed, etc.
I agree with Eric's (EJB69) comments about the flail mower verses the rear rough cutter. If I had to do it all over again, I would likely pursue a flail mower instead of the rear "brush hog" style mower (rear rough cutting mower) simply because of the ability to cut road sides a little easier. The rear mowers on these small tractors really can push the tractor around, because of their weight and the way it hangs off the rear of the tractor. You will feel it when on uneven surfaces, etc.
As far as putting the wheel spacers on and taking them off, while that may sound like something which one might consider, in reality, you won't end up doing it. It's a small project to remove and install loaded rear tires, at 150+ pounds per tire. While some do remove their rear tires to install tire chains in the winter, etc. planning to "swap" the wheel spacers on and off as needed is simply not practical. You will either find yourself leaving them on or off, depending upon the need.
Also, when you install the wheel spacers, they fasten to the axle flanges using the wheel bolts that normally secure the wheels to the hubs. The wheel spacers then have wheel studs, so lug nuts then fasten the wheels to the studs on the wheel spacers. It's much easier to mount the tractor wheels on wheel studs (on the spacers) than it is to hold up the wheel and tire and use the wheel bolts to get one started, to hold the wheel in alignment with the mounting holes on the axle flanges. You will see what I am talking about the very first time you remove the rear wheels and have to reinstall them with the wheel bolts, instead of having the axle flange studs to align and support the wheel, when installing it. In a nutshell, once you use the wheel spacers, you won't want to go back to using the wheel bolts to secure the wheels to the axle flanges.
You can make an alignment tool using a long bolt the same size as the wheel bolts and cutting the head off the bolt, then threading it into the axle flange to help support and align the wheel, but you surely won't want to be switching back and forth between mounting the wheels with wheel spacers on the machine and having the wheel studs on the wheel spacers to make the process MUCH easier, and then swapping back to removing them and having to use the bolts again. You will understand what I am talking about if you review the pictures of those who have shown the installation of the wheel spacers on their machines.
By the way, this isn't a John Deere issue, it's a common design issue with all SCUTS and CUT tractors..........Just as many European cars use wheel bolts to mount their wheels instead of lug nuts securing to axle flange bolts, the same applies to small tractors. Its the way they are designed. Bottom line, I doubt very much you will want to "switch back and forth" between using wheel spacers and not using them. Also, there is an issue where you need to keep an eye on the wheel bolt torque and swapping them back and forth only would add to the risk of a loose wheel.........
Main thing is don't over think the entire process. Just pick the machine and equipment and learn to use it safely and as productively as possible. Also, don't compromise either service or parts availability for a small savings on the purchase of any item. Make sure to confirm the parts inventory for any machine you purchase. As strange as it may sound, a close friend bought an "imported tractor" to save money over JD and he had to wait an extra week for the place that sold him the machine to order in service filters when his machine needed its first service. Also, he found out how poor the warranty coverage really is when they deny responsibility for a host of machine issues.
If you do look at flail mowers, make sure to check on the parts availability. Many flail mowers are made in either China or Italy and parts are not always easy to obtain as quickly as you may need them...........Also, there is a HUGE difference between a seller of the items and a servicing dealer of the same..............The dealer is there when you need them. The seller is likely only there when you want to buy the next item.................