has anyone any experience of both the 85 and 104 vw t5 1.9 td?

is there a great deal of difference between them in terms of power and mpg?

Not directly, but been vaguely looking at ’em myself.
Have a look over at

http://www.brick-yard.co.uk/forum/forums.html
and
http://www.vwt4forum.co.uk/index.php
– there’s a t5 section at both

(you’ll have to register for the vwt4forum iirc, probably don’t need to for brick-yard)

FWIW you can get the 85 remapped to something around the 104 (likewise the 104 for something higher) – there’s quite a few postings about it.