OK, neither you or the "one guy" is going to give us a clue which Motorola controller is being compared against, so I'll tell you that the Motorola 68HC05, 'HC08, and HC11 families would be roughly comparable to the various 8051 derivatives. A basic 68HC11 is most comparable to a basic 8051.

As far as the addressing modes go (for either processor), if you are programming in assembly language, the only important addressing modes are the ones for which you have developed a strategy to use. You would not give a hoot for addressing modes to facilitate stacked variable addressing if you always use direct addressing, right? If you are not prepared to use them, they are worthless (to you).

On the other hand, if you are programming in C, the only important addressing modes are those addressing modes (and data spaces) that the compiler supports. For example, an 'HC11 compiler I used many years ago refused to use the Y index register (plus offset) to access data on the stack, or anywhere else for that matter. A pity, since it generated extra code to do all stack accesses using the only other index register X.

So, what good is it to have 15 addressing modes if you or your compiler is only going to use 5 (an exaggeration) of them?