Should I purchase a Two Pack for myself or as a gift?

If you buy it for yourself, 1 license is automagically added to your account, much like any other Steam purchase.

The other becomes accessible in the "Gifts and Guest Passes" window (Games -> Manage Gifts and Guest Passes):

From here you can send the game license via email or to anyone on your friends list.

If you buy it as a gift then both licenses will be accessible through the "Gift and Guest Passes" window, and you won't be able to give yourself a copy.

When you purchase a pack from steam, one game is automatically added to your account and you will have the ability to grant the remaining gift key(s) to anyone on steam. So yes, you want to purchase it for yourself if you want the ability to keep one copy.