If you intend to use any smart PCAS (e.g. PowerFLARM) you want a Mode-
S, since suppressing your own transponder signal is a lot easier and
more reliable if the source is a Mode-S.
Other PCAS are also somewhat more likely to make sense of your
transmissions.
So do yourself and the World a favor and buy a Mode-S.

Ironically your decision is between buying into technology of the
fifties vs. the seventies...

Enjoy
Urs