It's a tricky one that, I would personally opt for larger OEM calipers and discs with some good pads, as then you can always upgrade to grooved/drilled discs later if you up the power a little more.
lets face it, who sticks with Stage 1+ forever? I won't be (despite trying to convince myself otherwise)!
Larger OEM calipers are heavy, but for ease of fitting and replacing parts along with the improved stopping power it's a good first step to take into the big brake world.