Well... I'm okay for either, really. It was mostly just I was planning to use Mega Diancie and I expected it wouldn't function on anything other than Offensive teams.Do you need it for Hyper Offense? If so your options for countering it are limited. There's Victini, which can switch in on anything, but can't handle any set with EQ, and is outsped at +2. Mega Scizor can switch in, but if you're using the most common (3 Attacks + SD) set, you can't recover, and you can't do impressive damage to it immediately. Mega Gyarados is nice, since in its normal form it can intimidate meta, soak up a zen headbutt, and boost until it can KO meta (after mega evolving into mega dos) with crunch. Scarf Landorus-T and Manectric both outspeed, KO, and intimidate, but they can be worn down easily, and the former falls to the rare ice punch + agility combo.
Btw, if you're using BO, there are many more options (that I won't list because I expect you're not).
Or maybe I'm completely wrong and it's the new star of balance