Sprint's everything mobile is definitely a good deal. If I were you I would try to keep every one together to take advantage of family plan pricing instead of multiple carriers.
if you were to decide to go go T Mobile I would seriously consider selling the Focus either private party or through Nextworth/Gazelle and switch your wife to the HD7. the Focus will run on T-Mobile once you SIM unlock but it will not have 3G speeds as the bands are different. Running WP7 on edge is not going to be enjoyable.
If you are set on the DVP, make sure you do some research to see how it is before buying because there are definitely reports of buggy devices of the tiny batch that has been released.