A loaded question because one has made the NHL and the other hasn't. Of the two, assuming he continues to develop, it has to be Reilly. He is a far better physical specimen at 19 than Gardiner and has at least a shot at developing into a player that does more than attack.
To those who keep saying both, how do you fit them into a lineup? Dion doesn't need to be babysitting a gambling partner, and it is a waste to have a high quality offense guy in the third pairing. I might pair one of them with Franson, because he has at least got size, but a Rielly/Gardiner pair would either be a bit too soft, or be spending too much time in the other teams end, or one player may have to back off of his key asset to support the other player's attacking game, so it devalues him.
However, it would not hurt to let them both play for long enough to establish that one is in fact the more talented player. It just seems for the long term three offensive types doesn't maximize the value of a high skill player, the same way that two does. sort of like center ice men.