Obviously, both power tube types have their pros and cons...you did a search already and possibly found my earlier posts on the subject, so I'm not repeating everything here, I'll just summarize my experiences:
Channel 1: 6L6s rule here, no question. More headroom, deeper and prettier bell-like cleans. To get a comparable tone from EL34s, you need to EQ heavily. (However, I've recently discovered I actually love the clean modes with EL34s when playing in a band - their improved definition allows me to actually hear what exactly I am playing in the mix, every single note in a chord, much better than the cleans of any other Mesa I own.)
Channel 2: EL34s rule here. Pretty dull and dead with 6L6s. EL34s give it some life, nice crunch and a much-desired British accent.
Channel 3: Both tube types work here, but the results will be slightly different. 6L6s give you more low-end thump and authority but their harsh and piercing top end can be hard to tame. EL34s give you more midrange grind and aggression and sweeter top end but their frequency response can feel narrower. I believe that the reason why some people prefer EL34s for Metallica tones is because some of the most classic recorded album sounds were Boogie amps slaved into Marshalls, which is easier to emulate with EL34s than with 6L6s.
You can't go wrong with either IMO. It boils down to personal preference. Decide which sounds are the most important to you, and use tubes that make those modes shine. Please do try both power tube types.
A good EQ pedal in the loop can often 'fix' the shortcomings of one power tube type, at least to a degree.
