[nas]

The 41C came out about 10 years before the 42S. Each would have their fans. The 42S doesn't have the hardware expand-ability but is generally faster and a bit more refined. If I wanted a nice handheld calculator today, I'd prefer the HP-42S or the Swiss Micro version. However, Swiss Micros have a 41C clone as well, if you prefer that.

[maxflow2]

The 42S was also mostly backward compatible with programs written for the 41C, intending to be its modern replacement. It did have an IR connection for some expandability (this was pre-USB era).

I'll also note that, with regards to using an HP42S today, most of their LCD screens have not aged super well and have faded a lot. Other models like the HP15 and HP32S(ii) have LCD screens that seem to age a bit better for whatever reason (potentially the support for the menu and limited graphing behavior of the 42S required some slightly different LCD parts). I have all these models, but wouldn't recommend an original 42S to anyone but a collector due to screen visibility issues - my HP15C and HP32S are plenty usable still though.

I also highly recommend the Swiss Micro replicas if you like RPN calcs. Very solid attempts at reproducing and modernizing some timeless calculators.