Have fun! If it's the outer, you only have to do what would be required to change a timing belt, and if it's the inner, you'll only have to pull the tranny. I say only because I'm being sarcastic.

Do yourself a favor and do them both-- otherwise, the one you don't change will fail and kill something. My Mitsu had the inner seal fail, pump 2 quarts of oil into the tranny, and kill gears 2 through 4.

If there's other maintenance you need to do (clutch, timing belt, water pump, etc, etc) now's the time. You're gonna have half the engine out, laying in the driveway anyhow.