I'm on flats these days, used to use clips, not very happy on them now because of a trashed leg. TBH there's not really that much in it, SPDs are probably "better" but not enough to override personal preference.
Best option is to use both and be proficient in both, they've both got advantages and things to teach and you can't make a good call between them til you've got good at both. (most folks "try" the other type for a couple of rides, never really get any good at them, then say they're rubbish)
<controversial> I think many if not most SPD riders could benefit from a few months on flats. Lots of people riding around very stiff and tall, it's not so much that flats encourage good technique, it's that they try and kill you if you do that This is the weakness and the gift of flats.