If you want to use it as a defender, you're better off using non-mega Diancie, since the mega version throws away big chunks of its defenses in order to get more stats elsewhere. Apart from having Magic Bounce, all of its mega changes are geared toward offense.
Stat-wise, both of its attack stats are identical. The only Fairy moves it gets are Moonblast and Dazzling Gleam, which are both special (indeed, there's only one physical Fairy move in the game, Play Rough, and it doesn't get that). Diancie also gets Power Gem as a special Rock move, but it gets stronger Rock moves on the physical side--including its signature move, Diamond Storm. It doesn't really get much in the way of off-type coverage, just Psychic and Earth Power on the special side, and Gyro Ball (which it's too fast to use effectively) plus a possible Explosion as physical. That leaves it hung up in a kind of limbo where it doesn't stand out much on either side, so it's up to you.