I think most shops will start developing production sites in D7 for delivery in Q3, assuming there are not huge delays.
That said, D7 is awesome and has some fantastic improvements.
However, If the client isn't asking for it, and if there isn't a major new feature required by the client that comes with D7... I would stick with D6. (An example of a major new feature required by the client built-in to D7 would be the need for having both public and private file systems or the need to stash uploaded files on an Amazon S3 server.