Yet another vote for new, from a secondhand-all-the-way person in most settings, mainly because of the warranty. And you don't know what kind of hard water a used one may have had to deal with.
We recently got our second ever new-to-us dishwasher, when we built our new house. One thing I learned (mostly by asking the salespeople, I didn't do any hard research myself) is that there are basically 2 types (as far as what happens to the stuff you didn't manually scrape off the plates). One type has a filter screen at the bottom that you have to manually take out and clean now and then. The other type doesn't, it has a sort of garbage disposal grinder that flushes it all away; less fiddly, but it can't handle things like paper labels on jars, so those need to be soaked off first. It helps to know which you'd prefer when you go looking.
