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.)
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.)