I just switched from platforms to ATACs on one of my city singlespeeds and it feels very good. I've never ridden road pedals, but the spds that came with my bike either didn't unclip at all, or came unclipped all the time. I'm still not clear on what float is either (how much you can rotate outward before unclipping?), but with a tight new (well, slightly used) pair of atacs and worn cleats I felt my feet turning all over the place, but never came near unclipping riding up Mont Royal a few times today - and I think that has more to do with worn cleats and foot position and riding style on this bike (road crank and singlespeed with a 70" gear on a mtn-type bike) vs the cleats and shoes being set up for my geared mtn bike. And not having ridden clipless for months.
I think the plastic ATACs have maybe better bearings than the aluminum ones. I have two pairs of plastic ones, both bought used on ebay for $35, and the bearings are smooth with no slop with no maintenance. They're heavier than they look though, my wellgo spd platform pedals are lighter than my ATACs. Not a concern for me, but maybe for you.
I'm a comparitive noob who never thought he'd go clipless mind you, after I got stitches from a 0mph crash onto a rock strewn road when I couldn't get out of the spds that came with my mtn bike. Atacs feel way better, and I really have no idea why some mountain bikers say they have a learning curve for how to clip and unclip - you snap in and you feel when you're in, and you clip out and know you're out, and I can't say that about my spds, who now only get their platform side used in the winter.