Difference in play:
Miller is a great isolation player; he loves to take his man off the dribble and has a deadly combination of crossovers and so on. He also has the size and foot work to develop into a very good post player for a SF. Not to mention he is shooting over 40% from 3, and is doing it with spot up jumpers and jumpers off the dribble (creating his own shot). He's also a willing enough, and often creative passer. So overall, a very complete offensive game, especially for someone his age. He has all the tools/abilities (except elite level athleticism) to become a deadly scorer in the NBA.
Barnes, on the other hand, could be criticized for his seeming inability to create his own shot. If you look at the stats posted earlier, his production dramatically decreases the more times he dribbles. Having said that, he is proving to be an incredible shooter. He seems to be able to shoot off the dribble, as well as spot up jumpers. From what I've seen, I'd say is decent playing with his back to the basket (not on the level of Miller, but there's something there). His passing? Well...it's almost non-existent. One thing I would say he has on Miller, along with shooting, is moving without the basketball. He has a high BBIQ and knows when to slash to the basket to receive a pass or go for an offensive put back.
Defensively, they have both shown good ability, imo, but I would give Barnes the edge in defending opposing wings at this point. However, Miller is probably the more versatile defender as he has the size and IQ in the post, that you could have him defend in the post in certain situations, and often did in high school (not sure how much of that he has done at Baylor).
Difference in potential:
Miller just seems to have a more versatile, deadly offensive arsenal of moves, while Barnes could be labeled has having too few dimensions to his game to imagine him as a top 10 player in the NBA. Though, I would say Barnes is the slightly more athletic of the two (though Miller is still recovering from an ACL injury, so we can't know that for sure). Overall, I would say due to Miller's advantage in size and versatility he has more potential as an NBA prospect, but more importantly, I see more of a fire in his eyes than I do in Barnes. I can see Barnes becoming a 20ppg scorer in the league one day (doing it within the flow of the offense), while I feel comfortable saying Miller has the potential to average 25ppg.