6L6's vs. EL34's...what's the difference...

and which do you prefer?...I only have experience with EL34's until recently when I got a Laney GH100L and the Laney takes both (with a switch for auto-biasing)-

I the 6L6's sound like they have more umph (low end) while the EL34's like to scream more to my ears...

what's the difference? which do you prefer and why?[/size]