Not enough difference to talk about!
Unless you don't reload! If you had a bunch of 17M4's and wanted another Cooper in a different configuration, I would certainly buy the 17M4, but if it is your first Cooper in 17M4/17FB, I'd go for the FB.
Forming brass for the 17M4 from 221FB brass isn't a big deal, and is a touch easier than forming 17AH from 22 H, but only because the headspace problem is a very little bit easier to solve.
Alex
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