Kevin Hayes:
2010-11
Boston College H-East 31 4 10 14 8
2011-12
Boston College H-East 44 7 21 28 10
2012-13
Boston College H-East 27 6 19 25 14
2013-14
Boston College H-East 40 27 38 65 16
Bill Arnold:
2010-11
Boston College H-East 39 10 10 20 38
2011-12
Boston College H-East 42 17 19 36 46
2012-13
Boston College H-East 38 17 18 35 40
2013-14
Calgary Flames NHL 1 0 0 0
2013-14
Boston College H-East 40 14 39 53 51
Comparable if you ask me. Factor in that Arnold is a VERY good defensive player, plays a much more physical style, and apparently is a high hockey-IQ player full of character - I will take Arnold every day of the week.
Hayes is bigger, but doesn't use it (look at his PIM for instance). Hayes is not as good of a skater, but he does have a better shot. You can argue that Hayes is a better offensive player, but that is about where the story ends.
I will be happy if the Flames sign Hayes - a free asset with potential. However, if it was a choice between Arnold and Hayes, I wouldn't even take a second to think. His game translates much better to the NHL. Watch those 3 players, and what you notice happening quite often is Arnold winning a puck battle in the neutral or defensive zone, throwing feeding Gaudreau the puck, and then Gaudreau and Hayes attack the net while Arnold trails up the ice after them. Arnold was the catalyst on so many plays. Hayes was a good option to have on that line - he has decent hands, a decent shot, and he isn't a bad skater (but something he really needs to work on for the NHL).
Hayes beat Arnold in scoring only in the last season, and if you watch those games, Arnold was time and again the 'unheralded' defensive presence that allowed Gaudreau and Hayes to attack more. Gaudreau was the biggest catalyst on that line, followed by Arnold, and though I won't say Hayes was just a passenger, he was the more easily replaced member of that line. They all had great synergy together, but Arnold is a much better prospect in practically every facet when compared to Hayes.