there are a million different ways to get your FCP project to DVD Studio Pro. I use compressor with the best quality setting. This is a 2 pass VBR MPEG2 file. You can configure DVD Studio Pro to do this with your quicktime files, but I use compressor because it is easier for me, plus I can use all the Mac's in my house to do the processing of the video via a qmaster node and a gigabit network.
I highly suggest the book mentioned above. THe manual to me was very confusing, but the book explained it in a way that was easier for my non engineer mind to understand.
A Quicktime Movie is just a wrapper. You can have an MPEG-2 Quicktime file. The Quicktime Movie quality is based upon which codec you use. An uncompressed, or well most any other better quality, codec will yield a better image than an MPEG-2 (.m2v from Compressor). But then that file is a data file, not meant for playback in a DVD player (even if it will fit onto a DVD). For a video DVD you need an MPEG-2.
Now if you're asking whether its better to take a Quicktime Movie into DVDSP or take an MPEG-2 into DVDSP, then its better to take the MPEG-2. If you take the Quicktime Movie straight into DVDSP and let it do the encoding, you're not likely to get a good result. DVDSP isn't really well designed for that. You want to use something like Compressor, BitVice, or Flip4Mac Episode to encode an MPEG-2 first, and then take that into DVDSP for your authoring. Good luck.