SPD! I dig flats for short rides on a non-fixed gear bicycle but for most everything else foot retention is way better then not having it. Toe clips and straps don't really do the job well when I need to get out of them because properly used they are tight on the foot and still use cleats (though probably way way less so now).

If you do go with flat pedals invest in a good set. Cheap plastic pedals don't last long and will give you headaches. Your top end bet would be White Industries pedals with Bruce Gordon Half Clips: Stainless strapless Toe Clips ? Nothing is better than a bike that fits All MUSA and super high quality and $40 bucks cheaper as a set. However you can find cheaper options and various high quality flat options.