ADIT stamp has a few drawbacks:
- It is only valid for 1 year. So you'd have to get it again
- Many times, you will be asked to surrender expired physical green card to get stamp. And this means, even if you get extension letter later, it will be useless as it cannot extend validity of stamp.
I'd get stamp if you urgently need it, but give some time because you never know how fast you can get InfoPass appointment.