Kopitar is almost exactly 1 year younger I think, not a big deal. No, +/- certainly isn't completely random, but it's only partially under a player's own control (which was the point). If he gets traded, or (two?) years from now moves on to a poorer defensive team (like Wheeler is on) as a FA, those numbers likely nosedive, although probably stay positive. That sort of thing is the reason that I think you can only project +/- out for a year or two.
Over the last 2 years,
Wheeler: 130 games, 47 goals, 63 assists, +1, 240 hits, 107 blocks, 10 PPG, 16 PPA, 354 shots
Kopitar : 129 games, 39 goals, 73 assists, +48, 160 hits, 69 blocks, 10 PPG, 29 PPA, 298 shots
If it's extended to a 3rd year, Kopitar evens out the goals and keeps the assist lead, but everything else basically keeps the same pattern. I admit that Kopitar does have a more extensive history of high end play. Especially considering Wheeler has been producing on a bad team, and Kopitar on a good team, I don't think it's unreasonable to think that Wheeler could be equally valuable going forward. If the Jets take a leap forward (and I do think they will), he could be more valuable imo. I don't think at the range they were picked, one is a fantasy killer while the other is an absolute stud, but I do understand people preferring Kopitar. I also admit however that, unlike baseball, I'm really just a casual hockey fan. I don't dive too deep into the advanced hockey stats, so maybe they say something different.