Very close, but not identical, if you're really paying attention to detail.
IMO:
1. indicates waiting until the event of going happens. Thus in this case you can/may say something/(comment/object) at the exact time you see him leaving/going.
They are pretty much the same but you could see a slight difference as Marius pointed out. It is very slight though so I wouldn't worry about it too much.
Imagine you are a kid hiding under a desk at one end of a big room with your friend. A teacher comes in to look (poorly) for you. As the teacher opens the door you say to your friend
Now I guess I've got it! Thank you nona. Anyway, what a fine subtlety! Wow!, and Wow again!
Don't you bother about me if I say that I'm pretty sure you're a lot more beautiful than that "photo" you've placed below your name for your "identification" here in EF?