I used both RCS and SCCS in the early days (e.g. 1985 - 1991). RCS was
what we used at Britton-Lee in the group that Eric Allman was part of.
SCCS is what we used at Sybase as it was gaining popularity. This was
so long ago that I don't remember all the details but I found that
RCS was much easier to use, especially in an environment that didn't
do much merging. Instead we used labels (or tags, I forget what they
were called) to mark which files were part of which release. Doing
this was much harder in SCCS, which contributed to the mess that
was Sybase software engineering.
Of course, all this could be explained by Eric's deep knowledge
of RCS, and the lack of somebody with Eric's knowledge at Sybase.
But, to me, an early adopter of source code control who wasn't
overly interested in speed, RCS was much easier to use.
Jon