This is how I see it as well. It depends on what you intend to use it for, and how many guns you want to compete with. I use my XDm 9 in both IDPA ESP and USPSA Production - and it is competitive if both, the problem is me. I also have the option to shoot USPSA Limited (if I don't mind scoring minor). A 9mm gives me options and costs less to shoot. You would have even more options with an M&P 9 as you could shoot IDPA SSP or ESP.
I like having options and I am cheap (plus have 3 teenage daughters, college expenses, etc.) so it works for me, for now.
Your results may vary.