For those of you who have had experience with both of these tubes, how do they compare to one another? What are the tonal characteristics? Which do you prefer and why?
I would not say they one is better. They are just different. It all comes down to what you want. Some amps well sound better with one or the other. There are lots of places on the web that tell the voice differences between all the common tubes.
I don't know about the "breaking up sooner" comment...I retubed a reissue '59 Bassman with KT66s (Groove Tubes back in '99) and it substantially increased the headroom...everything else was pretty much as described. It definitely gave the amp an overall bigger and smoother/creamier sound. I guess it ultimately depends on the individual tubes/amp.
It would depend on the tubes...a non-fixed bias amp should always be rebiased when changing tubes unless you are using tubes with a rating system like GT does (and even then I've found the matching to not always be "on"). When I changed the tubes in the Bassman, it required a significant bias adjustment.