EL34 and the 6CA7 are identical and often have both designations on the same tube/bottle.
I have never used KT77's, but have a lot of experience with EL34's. The reissue Mullard EL34 is a wonderful tube, just a hint of midrange bump which would make it a good jazz tube. If you want a neutral tube, go with something that is based on Telefunken tooling.
Jim