For our team, definitely Myers. I like Gardiner a lot, but he plays a very similar role to Rielly (left handed, great skating offensive dman), but with less talent. You would never put the two on the same pairing. He's also a bit too similar to Phaneuf - better skater, smaller/less physical, but still a lefty offensive catalyst type dman. Ideally, none of Phaneuf, Gardiner and Rielly are on the same pairing. Then you look at our top two defensive prospects, Finn and Percy, and they're ALSO lefties who are more offensive than defensive, and none of Phaneuf/Rielly/Gardiner/Finn/Percy seem like particularly strong potential d parterns.
Myers has been struggling a lot, but he's a 6'8", right handed dman with massive upside. I'd say his potential is higher than Gardiner's for any team, and he's a better fit for our roster, trading Gards for Myers straight up would be a no-brainer.