Denon 3910 or theta Carmen 2

I also own the Denon 3910, and was quite pleased with it in its "stock" condition. Then I added a Musical Fidelity X-10v3 tube output buffer (between the 3910 and my pre/pro), and the overall improvement in the audio quality was significant. Before you spend a lot of money, you might try the X-10v3. They typicall sell used here on A-gon for around $275-300 (altho' there is one currently for sale at $225). If you don't like the unit, you can always re-sell it. I can only state that I am VERY impressed with the sonic improvement the X-10v3 provides in my system.

I owned a Theta Carmen II and it was the best redbook transport I ever heard. I still regret having sold it in an episode of upgrade fever.
That said, I wouldn't make the switch you are proposing unless you a) don't really have much of an investment in SACD software. I don't personally think that the redbook play of the Theta exceeded SACD play from a good SACD player. And b) you are planning to hook it up to a really good DAC.

In short, if your primary and almost exclusive concern is the best redbook play you can get, go for the Theta.

Cheers.