I'd install cm7 then flash the nightlies... only because you never know if your phone is going to like the nightlies or there could be an error that you dislike. That way you still have a daily working phone rom.
Make a nandroid backup of your CM7 install after you get it how you like it, then you can flash the nightlies if you want and have something to fall back to if there is a glitch in the nightlies.