I don't think that there are any years that are bad, just models that you may want to avoid. All things being equal, any Bobcat with a Kubota engine would be my first choice. The big discriminators here are going to be the number of hours on the machine and the level care that the previous owner provided.
A lot of the larger Bobcats (8xx series) use Deutz diesels, which I am not overly crazy about. Like Tazza said, the 751 uses a Pugoet diesel (for some odd reason), which can be very expensive to fix.
I would also avoid older gas-engined machines. Once again, Bobcat used an odd assortment of engines in these machines, most of which (IMHO) were undersized/overtaxed for the application.
Also, most newer machines make greater use of electronics in their safety and diagnostic systems, which are a serious headache when they act up.
As for New Holland, in my limited experience (neighbour has an LS190), these machines are well built and well engineered, but may suffer from parts availability issues (may only be available from the dealer), due to engines, etc. being of their own design.
Hope this helps,
SR