What has the DM41 extra over the HP42s?

[rprosperi]

Answering that depends on what you typically do with calculators and what you plan to do and enjoy doing.

If you mostly write your own programs, and your interests lie in math calculations and engineering kids of applications, then I'd say the DM42/Free42 is the better choice. The DM42 has far better precision, is much faster, supports matrices and complex types natively and has some enhanced features for easily adding menus, etc. into your programs.

On the other hand if you prefer to explore other pre-written software, and/or are interested in dabbling with extending or modifying the environment (just to see if you can) then the 41X is probably a better choice. On the 41X you can use FOCAL (41/42 user code) but also Synthetic programming, which is artificial combinations of original commands, extended to do things unplanned by HP (somewhat akin to what the original hackers were doing in the 70s/80s) and also machine/assembler code.

The 41X ecosystem provides modules and other tools to accomplish most of the things the 42 can do (complex, matrices, etc.) but they are less trouble, more consistent and integrated better in the DM42, so you may as well use those if that's the goal.

hth