As a huge Fritz Lang fan, I strongly disagree.
M is a darling of mainstream film critics, but in my opinion it's absolutely atrocious, massively overrated, and not worthy of shining Metropolis' boots.
That said, Lang has some other masterpieces I would recommend, like Kremhild's Revenge and the Dr Mabuse series. I'd watch those long before touching M.
Back to those German reviews of Metropolis, I wouldn't put a lot of stock in to proto-Nazi Germany's discounting "sentimentality" in Metropolis. We know where hard-hearted disavowals of sentimentality led.