I was wondering if there was any preference here. I'm guessing members would lean more towards M.
While I enjoy Iain Bank's mainstream books I find the tone can be uneven from book to book and some (notably the latest, Steep Approach to Garbadale, took a while to get going and never really went anywhere when it did. It also seemed quite close to The Business in themes.
His touch seems more confident in his M books. I particularly enjoyed The Algebraist, but it's always good to see the Culture making a return.