I would guess in terms of raw compute efficiency the ARM will beat the eZ80 due to the older Z80 architecture (3x faster than slow is still slow, and the HPs used ARM9 IIRC, while the eZ80 is more similar to an ARM7 and only a 24 bit ALU, etc). However for a hand held calculator it’s unlikely to make much difference where hardware process, power management/clock domains and peripherals are going to be much bigger issues.
I think so, YMMV but the form-factor [1] of a calculator is beneficial. For example, if you are used to financial calculations on a HP 12C [2]. The calculator is small, is besides your keyboard, you just press a button (ON) to start it and a few keys combination for having a result, then turn it off. All while you, for example, are focusing on a document in your computer. Like a second or third monitor is complementary.
It is obvious that nowadays you can do the same in your computer but touching physical buttons makes also a different experience.
