Unfortunately I've burned through my stock of Sylvania 6CA7's...my Chieftain is hard on power tubes but nothing sounds as good in that amp.
Steve, to actually address your question:
The Sylvania's have more headroom, a bigger bottom end, and more pristine highs. I believe they also exhibit better power handling in that they last about twice as long as modern EL34's do in my Chieftain. They have less midrange than a good EL34.
IMO they make a mid focused amp sound bigger, but can make an amp with very strong highs and/or lows sound too boomy and/or shrill. My amp tech (Dave Sarge) said that for practical purposes, consider them a 6L6 pinned out for use in an EL34 amp...that pretty much sums up what I hear.