I considered the same thing in 99 and I ended up buying my 97 Jetta TDI.
The 1.9IDI's, available into 97 in the Jetta's, while not a slug, IS considerably less powerful. It is simpler, all mechanical, and can be upgraded a bit, like adding an intercooler.
Milage with the IDI is mid 40's as opposed to mid 50's minimum with the TDI (imperial figures) I get ~54mpg combined with my car & I drive very aggressively. Do you travel a lot of miles/yr, the fuel savings will help offset initial cost somewhat. Friend of mine with a 97 Jetta IDI gets mid 40's with spirited driving, and this is after he advanced timing, increased boost & fuel & added an intercooler, which overall increased his power & milage.
IMHO, if you can afford to shell out the extra bucks, go with the TDI, I spent darn near double what I intended to when I started car shopping but don't regret it one bit.
Definitely do a test drive in each of the IDI & TDI to feel the difference for yourself, and consider the TDI's power can be increased substantially for a moderate cost & minimal loss of economy if any. My milage went up after 'chipping', results vary.