I've owned the 50w EL34, and currently own the 50 6L6 and the 100w 6L6.
The 50 watters were near identical. The green channels were very slightly different, and the blue channel was looser on the EL34.
The 50w 6L6 vs the 100w, the 100 has less gain, but sounds fuller. That said, you can get 95% of the way there with the depth knob or whatever its called on the back of the 6L6. That, combined with it being friendlier volume-wise AND the fact it has more useable gain makes the 50w 6L6 the winner overall for me.
The 100w technically sounds a touch better, but the 6L6 is the better amp overall. I do have the 100w for sale locally, but I wont be mad if it never sells
Some call the 5153 "poor man SLOs", and they do sound pretty close. The SLO 30 I played was different enough to warrant owning both though, if I ever get the funds lol.
But yeah, the 5153s are great amps, more versatile than most give them credit for. They also have an inherent frequency-focus in the mids that you either love or hate. Most love it.