The OS really has to be dictated by the software you must use. I'm running Ubuntu at home and loving it, but all the home computer is used for is web, file storage, and watching TV and movies. Ubuntu is really great for all of this.
At work we use Autocad, and program PLCs, and XP is still doing this the best. we have a few Windows 7 Computers... but all of them have VMware XP systems installed to program the PLCs.
for 99% of people this is a true statment.