As a general note, both processes demand very careful control over
humidity and temperature. If you can't maintain a fairly low relative
humidity (not too low though - off the top of my head no more than 50%
or less than 20%, someone please correct me if my memory is off)
or a temperature of no more than 75 degrees F or less than about 60
degrees, then you'll never get consistent enough results to get a
good handle on either process.
As for which process is more messy, I'd say that gravure is. The
etchant (Ferric Chloride) is quite caustic, and of course printing
the plates involves the standard intaglio inking and wiping which
is much more messy (and fun!) than inking and printing collotype
plates, which is litho style printing.
When I worked with collotype, I was making my own emulsion and
"cooking" it onto glass plates. I've heard that others are getting good
results using a thick emulsion film instead. This would make the whole
collotype process much less work than gravure.
As for print quality, I'd say they were similar, though obviously there
are differences in the printed image owing to the greater thickness of ink
in the shadows of gravure images.
Hope this helps
Carson Graves
carson@ileaf.com