Hydraulic rear shock
Always better to change shocks in pairs. It keeps the car balanced correctly.
I have a 600 SEL and couldn't get original shocks for it so I changed to gas. I now find that the standard springs are too soft so I must change these also. I dont mind doing the work myself so cost is really only down to parts. If however you are going to get a garage to do it then best to stay with hydraulics I think.
Mad Englishman who like a challenge