My experieces.
We have owned and operated Case, JD, and Ford tractors for many years. Never had a Kubota so I can't say anything about them.
JD's are my favorite. They are generally built for WORK and will stand up to many hours of operation. We have a 6400 (85 hp) and it has almost 5000 hours on it with no problems and it still runs like a new one. We average 1000 or more hours a year on this one, as it has a loader and is a good general purpose tractor. It is used to feed round bales in the winter, mow pasture and hay in the summer, and anything you need a loader for. If you buy used, stay away from the 50 series. We had one for 5 years (bought new) and it was nothing but starting problems. Ran great and worked hard once you got it started.
JD has never built a tractor that will run hot on you. Can't say the same for the others.
We had a Ford 6610 (about 75hp) and it was not built for work like we use them. It ran hot all the time when you loaded it up with a 15' bushog. Had two engine rebuilds done during the 5 years or so we owned it. Traded it for the JD above and have never regretted the decision.
We have a Case 1070 and have had it for about 20 years now. It is large for most of what we do, but you can put a large disk behind it and open the throttle up for days at a time with no problems. We put 500 - 1000 hours on it a year.
We have a JD 1530 (40 - 45 hp) that about over 30 years old. Had a rebuild done back in 87 before we bought it. Still running great and is a great little utility tractor. It gets about 500 hours a year.
We have a Ford 5000. This tractor is a older tractor and it is really tough. Works all day and asks for more. I will be using it this weekend mowing pasture with a 15' bushhog.
You will probably never use one enough to need to do a engine rebuild. Something to keep in mind though, if you do, it is much cheaper to rebuild one that has sleeved cylinders than just bores. The older Fords that we had did not have sleeves. The JD's do.
Here are a few truths to tractor buying:
1) Buy bigger than what you think you need. A wise man once told me, "You can never get too big, but you can get too small". I have found that you can get too big, but it is all too easy to get too small... been there, done both, got T-shirt.
2) You get what you pay for. JD tends to be pricey, but if you use it enough, you will be glad you spent the money.
3) A loader is the single most useful attachment ever made. The last three tractors we bought have had loaders; the future ones will too.
4) If you are going to get a loader, it is worth the money to get 4X4 drive. I can't tell you how easy it is to be stuck once you get a load on the loader. This said, we have never had a 4X4 tractor for our loader tractor... .
Good Luck.