Follow-Up:
My 2000 Rialta (based on a 1999 Eurovan chassis) does not require premium fuel, works fine on regular unleaded, and has plenty of power/acceleration. The higher HP model is a plus if you're just buying this year, but I don't think it's so significant that it's worth trading up to it if you already own a VR-6 based Rialta. It doesn't appear that the extra hp was gained through major engine modification and instead seems to be more from tweaks to the existing 140hp VR6 engine. Of course this is just my opinion/observation and I'm not a VW mechanic.
Here is a link that might be useful: www.MotorHomepage.com