I'm currently studying computer science and for most of my life I was using Casio textbook display calculators (fx-85, fx-115ES, fx991EX) and during recent years I started to prefer using RPN calculators on my phone (real calc or emulators) after a failed transition to Ti Nspire CAS, which despite of having plenty of features it was very clunky and I happen to almost never use it unless I needed it for solving very specific problems like dealing with indefinite integrals.

I found using a stack very comfortable but I'm not sure whether 2 line display on hp 35s or basic display of DM 15L won't be limiting as I used to view 4 level of stack on my phone.

I Considered getting DM42 but it's steep price seems hard to justify, especially that I don't know how much I will end up using it at the end. Which RPN calculators would you guys recommend as a gateway drug and why?