DR Jason - Thanks.
...As soon as I said, "Yes", out loud, it just felt sooooo right.
The final part of my decision involved the people I was in the audition room with earlier. There was a respected director, a respected casting director, a composer, a lyricist and a playwright. When I mentioned that I was offered a job up at Williamstown, their response was not, "Oh, that's very nice, are you going to take it?" Rather it was, "Oh, you'll have a wonderful time up there. Congratulations!". -Like there was no question that I was NOT going to take the gig.
I talked to my friend, the director, for a bit, and he made a very good point: There are tons of people who apply to be a part of Williamstown every season. Only a few get the call. I didn't even apply this year - and I have in the past - and now I've been given the opportunity to get up there.
Carpe diem.