This isn't snobunny I was just too lazy to logout/login but here goes...
I've owned both, a 1992 Passat 16v and a 1999 Jetta GLS (Jetta IV). Your dad is probably looking for a much newer car, but I'll add my two cents as if it were still relevent anyways.
Its been stated, the Passat is bigger. Its not as quick, but it does WAY better on the freeway. My16v passat topped out at like 140 mph 5th gear was ridiculously long. The jetta starts to whine at like 80mph, but its quicker, handles a shitload better in a turn, the mpg is better around town, etc.
The other big difference that I encoutered was price. Passat parts for a B3 are rare and expensive. You might as well be driving an Audi. For real if you want a Passat just buy an Audi. If you want to mod anything whatsover on it brace yourself. One random example that sticks out in my mind is a Short shift kit on an MkII is like 50 bucks and installs in 5 minutes, on a B3 is like 400 bucks and you probably can't do it yourself. I don't really know much about VW's past 2000, but I loved both my VW's they were reliable as fuck and I only didn't buy a 3rd because I needed a truck. Good luck.