Having had both, I'd go with the 6550's. I do have a brand new one that I had specially requested with 6550's. I also have an older 2005 model if you're interested in that. I have KT88's in that and love them as much as the 6550's. Also, so I've heard, alot of el34's are poorly made with the exception of the new diezel germany versions. But with all the turmoil, my buddy had to wait over a month to get his el34's.
But then again, you can always switch back and forth. They're easy to bias. It really doesn't require a tech. It's so easy you could train a monkey to do it.