Most of the people on this board have Yanmars and so you are unlikely to find people who prefer the Kubota here. The benefits of the Yanmars is the price. They seem well made and durable. If cost is a difference, buy one of the more common yanmars and get implements and still save. Avoid rebuilt tractors as it it hard to know what exactly was done. If a good rebuild was done, it may be a good machine, if it was just painted then it wasn't rebuilt. Its just hard to tell the difference.
A used tractor is just that, used. It is more likely to break and has a somewhat unknow past. On the other hand, they are relatively durable, and not very hard to work on. Parts don't seem to be much of a problem, although there do seem to be some rumbles that yanmar is trying to cut off parts (due to lawsuits) so you might have to get parts directly from japan. Filters etc have crossovers to local auto parts so you can buy them locally.
A new tractor is new, it has a warranty, and a price tag to go with it. If money is no matter, buy the new tractor, but you'll end up 10k in the hole for very few additional features.
Finally, with most of the grey market tractors, you will not get some of the safety gear that is standard on a new tractor--No ROPS, no seat belt, and no pto sheild. The throttle moves the other direction.
So depending on your situation a yanmar may be better or a kubota. If you are rich or can sell your first born buy new, otherwise you will probably be almost as happy with a used yanmar.
