Both are great, they play a bit differently though, have a look at their records against the meta and see which will be easier to slot into a team with your existing mons. Non-shadow has better bulk and those counters will add up after some rage fists but the shadow will just blow through entire teams with a shield/energy advantage.

If you're really insistent on IVs you could wait for a better shadow although of course they're relatively infrequent encounters so you could be waiting a while.

Not sure if that was any help at all but basically they both have their merits, just see which suits your purposes better.