I like both, and they serve different purposes. My Rivera Quiana Studio w/ 6L6 have two channels, and Ch.1 is voiced for Brit tone. And, when I A/B the Rivera Ch.1 and Mesa Express 5:25 (EL84) the tones were very similar.
But, if I were to choose, then it'd be EL34s. That's why I love the RK II ability to have both!
PS. By the way, RK II pedal needs to have the ability to choose all 3 modes per channel. The new, flagship Marshall JVM pedal allows just that!