The B5 has less room in the engine compartment than a B4, so if you want to go V8, there is less room. There is also less rear seat room in a B5 vs B4.
The front suspension on a B5 is a pain with lots of parts that need replacement. The front suspension on a B4 is simple and you can replace bushings and ball joints and have a new front suspension. B5s usually go up for sale when the suspension is shot, and they don't want to spend the money to refresh.
Rear suspension is similar to B5 so parts are not hard to come by. B4 has a rear diff lock where as the B5 doesn't.
The 1.8T B5 was a popular car, but a manual one has probably been beaten to death. The auto transmission ones are probably not beat, but who wants a auto.
A V6 was mention, but not much difference between the v6 in either model. Both slow and a lot of front end weight with a cast iron block.
Which engine compartment would you rather work in.
B4 with 1.8T Thank you Dave for the use of your picture.
B5 with 1.8T
You can see the thread here for the install of the 1.8T in a B4.http://theprojectpad.com/viewtopic.php?f=11&t=834