The assemblers only differ as far as simple syntax, and even then not by much. Making apps is extremely similar to normal programs, with just a few extra steps.
I recommend Brass. It's a more modern assembler and it's cross-platform.
For making apps, as I said, there are a few extra steps. This means compiling to hex code in the right format, giving it the right header information, as well as signing the resulting output so calculators will accept it as a valid application. Brass has directives for all of these, so it's not much more complicated than making normal assembly programs.
