I found that the M costs you and extra $6k for similar cars. I found a 2007 3.0si, manual, Monaco Blue, tan interior, 9,400 miles, premium + sport package but no nav for $27k, which seemed about right for that car. I found a 2007 M, manual, Monaco Blue, tan interior, 10,100 miles, every option except heated seats and headlight washers for $33k. The M stickered at $61,400. The 3.0si was $46,800. The M has depreciated 46%, the 3.0si 42%. Based on hours of research on autotrader, they seem to drop equally as the miles go up or they get older. And the separations seems to stay at $4,000 - $6,000.
So, your assessment is correct. You can either save money or get a nicer 3.0si vs. buying an M. I was ready to make that trade and made an offer on that 2007 3.0si. Then my wife asked me, "If money were no object, which one would you want?" I said the M. She said I don't get two bites at the mid-life apple, so pick the right car the first time. I did. She was right. But that's my situation.