Whats your thoughts (pro/cons) for touring with either SPD or Flats?

For me personally I have almost no experience with flats! Nearly all my cycling has been done with SPD and TBH I like the idea of flats but am so very use to SPD now.

My concern is not easy riding but when the going gets tough I really appreciate SPD's for peddling circles rather than squares!

Please let me know your input as for the reasons why you use either types? Especially for long distance touring for a long duration of time.