Ivan-
When most think poker they think poker room not sportsbook. The main reason Party was chosen is that many of the folks interested in playing have accounts there and you can set up a private room with your own password in less than 10 seconds.
With regard to playing at a sportsbook, I would play at WSEX since I have both a book and poker account there but would rather play at Party.
BTW, in your post you mentioned that a few players have fewer players than Party, why would that be attractive?