> One thing that has always befuddled me with HP calcs is their model numbers that don't seem to follow any rhyme or reason.
Back in the day, HP had nice sales literature that described the differences between the models quite nicely.
> And how does -48 compare, besides the relatively useless graphing capability?
The -42 and -48 were both intended to be successors, of a sort, to the HP-41 family. The -42 It was less expensive than a -48, and placed more emphasis on a function-per-key style of operation, as well as -41 compatibility. (The -42 was program source-code compatible with a nicely loaded instance of a -41.) Where the -42 suffered in comparison to the -41 was that it did not offer any kind of expansion slots, and, unlike the -41, the -42 was never intended to be the 'top of the line'.
To understand the -48, you need to look back to the -28 (as well as a couple of similar business models). After developing the -41, HP realized that they needed a new higher level internal programming model to take the calculator line to where they wanted it to go. In reaction to their growing ambitions, they developed a language for their calculators called RPL... Reverse Polish Lisp. There are a few examples of RPL in this thread, but if you think of a dynamically typed type safe variant of FORTH you wouldn't be far off from RPL. It was RPL that brought features like symbolic algebra, graphics, and a >4 level stack HP's product line, and it was the -28 that was the first scientific instance of this platform. However, the -28 was a bulky clamshell design, behaved fairly differently from a -41, and was sold in parallel with the (still popular) -41 series.
The -48 was a more spiritual successor to both the -41 and the -28. The -48 unified the new programming model of the -28 with the non-folding portrait style casing of the -28. The -48 also added a serial port and a couple expansion ports, so it carried forward some of the -41's legacy of expandability.
> I see lots of people also preferring some other models (15, 16, 35, 41 etc), which I would have imagined to be strict subsets of these "juggernauts", but maybe that is not the case?
I have an -11, a couple -48 variants, and a -42 emulator on my phone. While the -48 is strictly the most powerful, it can also be a bit cumbersome to use. The smaller platforms have their place.