Bicycle Mechanics - Differences between Derailleur upgrades?

I have two road bikes. One has Shimano 105 and the other Ultrega. Supposedly the differences in upgrades is between materials, weight.? My Ultrega does shift better. Like clockwork.
Other than the qualities mentioned, is there really any differences between say, design? Anyone found a web site or other reference sources explaining these differences?

Shimano has 5 grades of derailleurs. That allows them to cover a pretty broad price/quality range. As a general rule, as you move up the food chain, all of the components get progressively lighter in weight and operate progressively more smoothly.

Truthfully, I don't think that I can subjectively feel much difference if I go up or down one grade level, but I can definitely tell if I go up or down two. Most of the difference is in the materials they are made of and the manufacturing methods which result in a better finish. Finish is more important than just cosmetics because any moving part that has burrs can hang up momentarily then release suddenly which results in degraded performance.

OK, here goes:

Sora=cr@p
Tiagra= slightly better
105=basic quality stuff
Ultegra (600, too)=high quality stuff
Dura Ace=racing stuff

There are differences between grades that you do not see-such as metal washers rather than plastic, etc.

You'll find what perhaps is your answer on this page:
http://www.bike.com/template.asp?date=9%2F5%2F2001&lsectionnumber=6

The specific Shimano line comparisons / differences are listed in the lower part of the article. However, you may find the entire article interesting as well. NOTE: The author has a slight Shimano bias so Campy owners be mindful.

Livingood.. Thanks for link. That information has always been what I was wondering about.