inside of the ball would be optimal.
you can hit the outside of the ball if you don't have the mechanics down yet as it's safer.
But I would work towards hitting the inside of the ball otherwise you might end up just using a 'down the line' stroke for your inside out shot.
Yonex VCore 100s - SW 351 6pts HL
Tour Bite / Luxilon 4G 16 @ 51lbs