EH's will have more high end energy. Ditch thoughts about Svetlana 6550's, they are Sovteks in desguise.
The SED "winged C" (Old Svetlana's) are IMHO far more neutral in tone and very rugged. The only caveat on the SED's is to be sure you get those with rectangular holes in the plates, not the really old ones with round holes, AND, since there have been a few 'hot' ones you take less of a chance of tube run away if you have an amp which uses manual bias as opposed to those with auto bias set ups or limited bias range. I've had one such tube I used in a Primaluna amp which has auto bias and the amp had to be sent in for repairs to the auto bias circuit.
The SED "winged C" (Old Svetlana's) are IMHO far more neutral in tone and very rugged. The only caveat on the SED's is to be sure you get those with rectangular holes in the plates, not the really old ones with round holes, AND, since there have been a few 'hot' ones you take less of a chance of tube run away if you have an amp which uses manual bias as opposed to those with auto bias set ups or limited bias range. I've had one such tube I used in a Primaluna amp which has auto bias and the amp had to be sent in for repairs to the auto bias circuit.