A quick survey of reviews by non-scientists suggests overwhelming agreement with the statement in Wikipedia that Dante's Peak is more scientifically accurate than Volcano, although Wikipedia only says "slightly". The impression is strengthened
by the careful discussions of the science of
Dante's Peak that have been published by the USGS and the UMD Geology department without bothering to mention Volcano.
However, the only side-by-side comparison between the two movies that I could find made by an actual geologist comes out differently. Erik Klemetti's guide to volcano movies has heavy criticism for both; on scientific accuracy, Volcano scores 4 out of 9 but Dante's Peak gets only 3.
In fact, most of the volcano movies he discusses score the minimum 1 out of 9 for science; the notable exception is the made-for-TV Supervolcano (2005), which gets 6 out of 9.