I just changed my amp from EL34's the 6L6's. Personally I'm in love with the 6L6's, but it depends on what you play. I play slow heavier metal, i.e. Neurosis, and my guitar is tuned low, B standard, which 6L6's respond a lot better to than EL34's.
Easiest way to break down the difference, and this is by no means a perfect description:
EL34's = a more 'British' amp tone - think Marshall
6L6's = a more 'American' amp tone - think Mesa Boogie.
Hope that helps :-)