If you have a substantial amount invested in CD's it makes sense to upgrade to a good CD player. Chances are, even if one of the new formats catches on, still an iffy proposition, you won't repurchase every title you own on CD in the new format, particularly if software prices are higher. A good redbook CD player will make good music for you for years to come. Both of the players you mention have great reps, I can only speak to the Rega, which is a very musical and warm sounding player with pretty good resolution. I'm sure the Theta is a good player too, and perhaps others can comment on it.
Happy Listening!
Happy Listening!