I have a really hard time picking which one I prefer, simply because of the range of tones you can get depending on what kind of amp is using either tubes.
I was absolutely in love with 6L6s for a while, preferred the tone of my '68 Twin much more than a Marshall with 34's. That said, I eventually bought a Matchless Chieftain that runs the EL34's class A. Phil Jamison definitely builds his amps in the British tradition, but the Chieftain kinda brings the tubes' tone much closer to that of a 6L6, American-ish tone. A little more bottom end and overall air. I still have these two amps, and couldn't part with either.
Now if you get into small bottle tube comparisons like EL84's to 6V6's, I think the latter wins without questions. EL84's sound like piercing, dry glass to me. The irony is that I've heard so many great records recorded with AC-30s, but I've never had a really great personal experience with one.