I am looking at buying one of 2 used tt's  a 2001 and 2004, both 1.8.

Are there known differences in reliability between these (or any other) model years ?

Or if I am to buy a used tt - is one model year preferable to others, or one to avoid ?

(eg I have a '85 alfa spider, which is preferable to the later model years mechanically)

I love my 01, but an 04 is probably in general a better buy.  It should still be under the extended warranties, for one thing.  If you have to replace the cluster or timing belt out of warranty, that can be very expensive.  I do think that some of the electrical/annoying problems are a little more common in the 01's.