Honestly, either or would be a good option to power up. Groudon has high base defense (250), so having a 14 defense IV versus 15 will likely make no difference whatsoever.
On a similar note, when maxed out, the difference between your Groudons in terms of HP is 1. The one with 13 stamina IV will max out with 154 hp, while the one with a 12 stamina IV will have 153 hp.
So you can't really go wrong either way as far as I can tell.