Choose one. If you control a commander as you cast this spell, you may choose both.
• Target player draws X cards.
• Target player mills twice X cards.
Controlling a commander means that you control a permanent on the battlefield that is someone's commander. It can be yours or another player's. Controlling a commander spell on the stack will not allow you to choose both modes.
You may target the same player with both modes or target different players. The effects happen in the order they're listed on the card.