Both RAW and Jpeg. Different tools for the job but most of the time Jpeg works just fine.

I don't see any reason to shoot a TIF, basically the same as a Jpeg but without the compression. To me, the biggest benefits of RAW are the larger bit depth and adjustable white balance (in that order, too). As long as you're shooting Jpegs at the highest quality setting you'll have a high quality file - and a TIF is a huge file, even larger than RAW and doesn't have the above two advantages.

I also use Mpix but have been sending them Jpegs for printing (saved in Photoshop at a quality of 12 which is the highest). I save the finished file as a TIF and convert to Jpeg before sending them (it's quicker and no real-world loss of quality).