No thoughts on Chicago, as I've never run it. But I have done NYC 4 times and I had a blast. Yes, you were in a sea of people, but the crowds were just AMAZING! And all of the other runners from around the world really give that race an international flare.
That being said, NYC is NOT a race to expect a PR in or to BQ. Just too many people. And at a 3:45 you would be amongst a LOT of friends.
So I guess it really just depends on how much of a crowd you can handle. There is a big one at NYC. But the crowds on the sidelines are AMAZING!!!