To me it really comes down to a single question, would you want a rounder more american clean tone or a more birtish upper mid gain tone?
I only have the EL34 but also have an Ecstasy classic. The 20th EL34 gets closer to a fender style clean than the classic but not as good as the 6l6 vids and clips I've come across. The gain channel has loads more gain available than the classic and certainly sounds more marshally or british with better upper mids than the 6l6 vids and clips. While the EL34 doesn't sound exactly like a fender it still has a very great clean tone and the gain channels or more to my taste than what I've heard from the 6l6 vids and clips so its EL34 for me. That being said Reza's demo of the 6l6 sounds pretty damned great... I think most anyone would be quite pleased with either because they are monster amps but if you answer the opening question it should tell you which one may be better for you.
Vs. and Ecstasy classic, the 20th has more available gain and the additional excursion and presence controls really let the user dial in tones for each channel in a better way I think...