Let me tell you a little story. I am not sure how much it will help, but it is my story.
I bought a 5hp MTD front tine tiller about 12 years ago when we lived in the bay area. We had a 6000 sq ft lot and a 20x40 garden. I used that little MTD tiller to till up the garden, the back yard and the front yard. That little thing bucked, kicked, screamed and was a monster to hold onto, but it did the job. I rented a sod cutter for the back yard (the bigger of the two). I lucked out on the front yard and was able to slip the bobcat operator who the driveway contractor hired to remove the old driveway 20 bucks to have him scrape the front yard. I was in my mid 30's then and holding onto a bucking bronco was an interesting experience then. I put in raised beds in the garden after a couple of years and the 5hp front tine tiller could be lifted into the garden to till in amendments every year.
We moved to our new house about 4 years ago. The ground here is good old california clay, similar to what we had at the old place. The former owners had a 40x100 garden are with an 8x16 shed in one corner. I tilled up that garden with the front tine MTD the first year we were here. It bucked, kicked, jumped, twisted and was generally a bear to hold onto just like before. It did the job but I paid the price. I was sore for a week.
The next year I was certain that having tilled in a bunch of compost it would be easier to till than before. Boy was I wrong. An hour into the bucking process I called it quits. I went down to the rental yard and hired a 10hp rear tine tiller. 4 1/2 hour later I had managed to complete one pass at a minimum depth. The depth adjustment was finiky at best and the durned thing was just as bucky and hard to control on the hard clay as the front tine model. I got the garden in and the gophers and bugs ate it all.
Eventually I convinced myself I needed a tractor. I really did need it. After all, we all DO need our toys, don't we? There were only three essential implements on my "absolutely must have" list. A front end loader, a box blade and a tiller. I have since added a post hole digger. Tilling the garden the spring after I bought the tractor was an absolute joy. Get up in the morning, drink my coffee, argue with the 3pt hitch for a half hour removing the box blade and installing the tiller, start it up and go for a little drive. Tilling the entire garden took a half hour. Just engage the PTO, lower the tiller into the ground and feather the hydro peddle. I sometimes have to jump on the clutch when it bogs down. Other than that, it is just move along at a half a mile an hour while the machine does the work. The tiller is bucking, jumping, wiggling and twisting just like every other tiller I have used, but this time it is trying to beat up a 2000 pound tractor, not a 200 pound me. It is also being turned by a 18hp diesel motor instead of a 5-10 hp gasoline motor.
I still have the 5hp front tine tiller. I use it for final bed preparation even today. I break the ground and till in most of the bulk amendments with the kubota, mark out where the growing beds will be, and do a final till with the front tine tiller to clean up any compaction caused by operating a tractor in such a confined space.
You don't say what is where you plan on putting your garden. If it was formerly grass you will probably want to remove the grass. I know people who have had success simply spraying the lawn with roundup and tilling the grass into the ground, but most recommend scrapping off an old lawn before starting a garden. If you do have to scrap off an old lawn, and you have no need for a compact tractor, it might make sense to either rent a tractor with a loader and tiller for the initial preparation, or pay someone to do the job. The reason I say a loader is to spread the 15+ yards of compost you are going to be turning into the ground assuming a 3 inch layer. You can probably get away with less machine for subsequent tills unless your ground hardens up as bad as mine.
The tradeoffs you face are cost, capability and manuverability. You can get a smaller tiller further into corners than a bigger tiller. My garden is fenced I can get the tractor based tiller get no closer than about a foot to the sides and two feet to the ends. I have to clean a bunch of it up with the small front tine tiller. This would not be an issue if your garden is to be in open space.
I love the 3pt tiller on the kubota. It makes garden prep a snap. I broke the ground and spread 3 tons of mushroom compost (1 1/2 inches) in a matter of a couple of hours. Much of that spent shoveling the compost out of the back of my pickup and into the FEL. I probably would have spent half that much time if I had a dump bed in my truck. But, it is definately overkill for what I do, and it would be overkill for your needs. You could do everything you need in your 40x60 garden with a shovel and a WHOLE LOT of work, a front tine tiller and a LOT of work, a rear tine tiller or a tractor tiller.
While I myself have no experience with them, I have considered one of the "walking tractors" for when I get rid of the kubota. They are basically motors with multiple attachments. They make tillers and trailers and slashers that all plug on to the same motor assembly. That might be something to consider if you intend to "overbuy".