@youdruk,
Hello, Youdruk:
1. One of my books gives these examples:
It is nice SEEING you.
It is nice TO SEE you.
It says that there is not much difference, BUT the -ing may refer to an action; the to- may imply "in general."
*****
2. Another book says that:
a. -ing is "more immediate, more vivid."
b. to - is "more remote, more objective."
THEREFORE, if you are really, really happy to be at the side of your friend, you might consider:
"It's REALLY nice to be SITTING here with you!"
James
My references:
Longman English Grammar
The Grammar Book