I have appreciated miz's mic work lately. I really like Owens for his in ring work. I actually feel like they've tamed Owens Arsenal a bit which pisses me off. The moves he pulled in the Cena matches were incredible for a guy who looks out of shape, lol. Haven't seen him flip off the top rope in a long time.
Definitely Miz. A heel's job is to get the face over and we've seen him do that so many times. Ziggler and Neville are actually less over after feuding with Owens. Which happens when heel smark darlings split the reaction of every match.
Miz is the best heel because he gets actual heat, people really want to hate him. Nothing against Owens I think he is good too and can be a good heel but he is beloved too much by the fans, a real heel should be people like Jake the Snake, Triple H, Rick Rude, Ric Flair, people who can really make you want to boo them, be it annoying or just plain nasty.
Now most of that isn't Owens fault. But if fans cheer you, you are not a good heel even if you are portrayed as one. Is the same in reverse, its like saying is Cena a good face, the answer is no he isnt because he gets a heck load of boos everywhere he goes.
Now this could also be partly down to the current generation and/or the WWE's inability to make a good face and so the heels come off looking better. So horrible booking or typical WWe audience, you decide
Easily The Miz. Owens may be superior in-ring (although the jury is still out on that, purely based on his WWE work), but Miz has the ability to make everyone just wanna punch him in the face, which is the most important factor for a heel. It´s even more remarkable when you take into consideration how long it´s been since Miz was used for more than PR stuff and the occasional Miz TV.
In ring work doesn´t matter as much, especially not for the type of heel Miz is playing (the delusional coward)
So far, I'd say The Miz but he has an easier job doing it since he actually gets to have mic time and wrestle matches with purpose while Owens just comes to to fight in meaningless matches. Give 'em both the same sort of treatment and evaluate their performances after the fact.