All <M,U> all the time. I always try to find M' turns in any alg. F perm? Got an M'. OLLs? A good portion have M'.
I also have a nice fingertrick for M. I can sub-1.5 H and Ub and sub-1.6 Ua. Z is teetering at 2. <R,U> H perm is ridiculous, but for all other perms add .7 seconds and that's my time with <R,U>.
But which ones are better? <M,U> H perm is the fastest (Rw2R2 or L2Lw2 = M2 in my book), but for the others it is a matter of taste and practice.