I'd say the way the menus are implemented in the HP42s is more useable than trying to cram things onto buttons. For example, on the HP12c you have to remember

I'd say the way the menus are implemented in the HP42s is more useable than trying to cram things onto buttons. For example, on the HP12c you have to remember which registers the sum and mean of x and y are - usefully it tells you on the back. The HP42s is more explicit with its soft menus.