OK, I'll jump in on this one.  I've made a bunch of 5x5s and used them for marten, mink, and raccoons.  They're deadly on all three.  For 'coons, the big thing is avoiding triggering with the paw.  My traps have a striker bar on them and they will hold a 'coon by the paw but there's too much room "under the jaws".
To avoid paw catches, I put the trigger on top on the inside jaw and bend it so it's almost horizontal.  I also have holes in the tops of my boxes and wire the bait to the top.  Occasionally I'll have him caught by a paw and the head, but he goes down quick.  My springs are made of 3/16" music wire and the striker breaks the spine about half the time on coons (always on mink and marten).  Deep boxes, mail boxes, or buckets are a big help to reduce reaching.
As far as 160s vs 220s.  I think 160s kill 'coons quicker than 220s.  Like RayA, most of my 160-caught 'coons just fall down.  I think 220s catch them a bit further back and they don't get stunned like they do when the 160 (or 5x5) raps them in the base of the skull.  That being said, I got a lot of avoidance trying 160s in trail sets- even when they were lifted up off the ground.  Now I just use buckets or boxes right next to the trails.  They rarely seem to get past the bucket. ;-)
I bait my conibear sets with marshmallows and sweet baits/lures to avoid non-targets.  I save the muskrat meat or fish for the dirtholes and pockets with footholds.  So far- so good.  34 seasons with no dogs caught.
John