Comet and Turbomon for the SAM Coupé make the ideal development package for m/c projects, but ... everyone deals with Windows or Linux, even Comet source files we can assemble by use of a python script or a development tool (a complete IDE) written in Java.
The myth says, the author of Comet cooperate with the maker of Tornado. Both are valuable pieces of software, but the most speccyal assembler is the Prometheus - see the kind of writing commands, by pressing one key. Some people adorate Memory Resident System (MRS), many hate it.
Every assembler with linenumbers in source is obsolete. The old good Pikasm looks professional and handles conditions (IF-THEN-ELSE). Comet can handle sourcefiles over 400K.