They're products of different eras - ten years separate the two. The 41 is almost pre-personal computer. By the time the 42 rolls around, personal computing is already well established. This is particularly true in the market segments that were buying calculators.

HP was also working to rationalize its product line at the time. The 41 was introduced as the high-end offering, but a few years later, they also introduced the 28C/S. These were intended as next-generation offerings. The 28 was arguably higher end than the 41, but it wasn't expandable and had a more cumbersome UI. For a few years, this left HP with two disjoint high end offerings.

To address this, HP slotted the 48SX in as a partial replacement for both the 41 and the 28. (Hence the name). The 48 improved on the 28 a bit, switched back to a traditional handheld calculator form factor, and brought in the expansion and bidirectional I/O from the 41. But it still had the more cumbersome UI of the 28.

The 42 was slotted below the 48 as a more direct replacement of the 41. A very similar UI to the 41 but built on the newer technology. A few years after all of this, the entire business essentially shut down. (See my earlier point on the emergence of personal computing, and particularly laptops. The calculator niche narrowed down to essentially the vanishing point, outside of education.)

Where this left the 42 is as the last and best of HP's traditional calculators. It's prized because of both the capabilities, and the lack of expansion doesn't matter that much. There are better options for that.

This is a long answer, I know, but the short story is that as a practical tool, the 42 is probably the better choice these days. (Although if you need something like an HP-IL barcode scanner or something, you need a 41.)

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.

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.

While I own a few HP calculators, my favorite is still the HP-28. Not sure why it isn't as remembered as the other models, but it's the one I still reach for whenever need to calculate anything.

> While I own a few HP calculators, my favorite is still the HP-28. Not sure why it isn't as remembered as the other models,

I've been a big 48 fan for years. (The 48 and the 28 are very similar.)

I think they both suffer a bit from design overreach. The devices are very capable, but the capability can make them more cumbersome to use than earlier, simpler devices. And then the 28 has the problem that with the flip out case it isn't really a handheld.

The result is that compared to contemporary calculators they were more difficult to use and compared to contemporary computers they weren't nearly as capable. So they fell into a gap, and there wasn't enough life left in the calculator market left to fund a way out.

Another way of looking at this is that the 28 was a v1 product, the 48 was a v2 product, and there wasn't ever the v3 product (where v3 is usually where products hit their stride). I don't really count the 49 or 50 because by that time, the calculator division had been radically restructured and those products were the output of a very different kind of engineering process. (No offense to Jean-Yves Avenard, who did great work within the constraints of the time.)

The flip case is a large reason why I prefer the 28 though. When I just need a calculator, fold it back and it's a handheld calculator. When I want to program, fold it out like a book and have a more extensive keyboard. Best of both worlds.

I should find a 48, don't have one of those. I have a 50 but don't like it much. I keep it in the office for basic use.

> The flip case is a large reason why I prefer the 28 though

I can see the split case might have some benefits. It wouldn't work the way you describe, but I wish they'd put scientific calculator hardware in the 200LX case. Nice display and great keyboard. (The 200LX _did_ have a built in program that acted a little like an 18C business calculator.)

> I should find a 48, don't have one of those

I have both a 48SX and a GX. I like the form factor of both better than the 50, although the hardware on the 50 is much better. (If I remember properly, they replaced the custom Saturn CPU in the 50 with an ARM chip running an emulator... it's overall much faster.) Neither of mine get much use, but they do have lots of sentimental value.