Jack,
I own both the 2910 and 3930ci. I upgraded after reading Clint's review of the 3930ci. I believe PQ will depend on your display. I have a sony 60" sxrd and there is an improvement, tho it is not evident on every scene. In the beginning of "Gladiator", when Maximus is walking thru the field, the hairs on his hand are more detailed, as is the fur on his robe moments later. In "Braveheart", the scene just before the English soldiers abduct Mel's bride, the grizzelled old soldier says "look lively sargent", the englishmen's armour has such depth and detail it seems to pop off the screen, as near to 3-d as i've seen. But, that said, I don't think these differences are night and day, for the 2910 is a fine performer. One of the main reasons I chose the 3930ci (along with a 3806 receiver used as a pre-pro) was to take advantage of the denonlink feature, this would allow me to add a blue-ray or hd player to the ext. in jacks at a later date. BUT...after reading other threads on this subject i connected the 3930ci to the ext. in on the 3806 and SQ is outstanding. I think this is where you may find a bigger difference between players. On Rickie Lee Jones first cd, the bass intro to "easy money" has more texture, more detail as his fingers move over the neck of the bass. On "night train", which is a great demo song, the way the guitarist slides his (or her) fingers over the frets seems more pronounced, you can feel the skin scrape the strings.
I'm like you Jack. I have about 400 dvd's and about 60 dvd-audio and sacd's, and i'm sure to add more between now and when the hd formats reach the point where i would feel comfortable buying either option. This, hopefully, will be the last sd player i will own and i think i'll be happy, as i think you would. That said, your original question was is it worth the extra money to upgrade? Let me say that i think i could have lived happily with the 2910, but after having the 3930ci to play with, i'd feel like i was missing something if i went back. If you could get a dealer to let you try one for a few days you could decide.