For men (like me) I believe the choice is easy -- definitely inside the blazer. Otherwise you'll look like disco stu (for men).
For women, I think both look great in or out. There are some tips I would give that make it look good for women:
If you button to the top, with top button closed, then obviously collar inside. It would be too hard to even try and get it to stay outside.
For one button open, it is probably better to keep it inside too for the same reason that it would probably slip back in anyway.
For 2 or 3 buttons open, I think putting the collar outside looks better. Collars are beautiful and what better way to accent it by putting it outside, if your shirt is open.
For 4 buttons open (as some celebrities and musicians wear), it is better to put the collar inside because otherwise the shirt will open too far exposing the bra and most of the breasts.
If you pop the collar up, then you get the best of both worlds, not quite out on top of blazer and not quite inside.