Confessing should be done in real life only because:

1. You can look into her/his(if you swing that way) eyes, which means that you truly mean what you say.
2. If you confess IRL, she/he will A) Run away [Failure]

B) Kiss you.
If you get B), enjoy your new GF and have a happy ever after!

So yeah, confessing IRL will give you a straightforward answer from your chosen, so there won't be any beating around the bush.
Plus she may admire your courage. [Unless you start to cry like in Densha Otoko, jesus christ. I <SOBSOBSOBSOB> LOVE <MORE SOB> YOU. When you get to that I<HEART>YOU part, it isn't supposed to last 4-6 minutes, damn it!]

But then again, lol I am a teenager. What do I know, I haven't even had a girlfriend.