I think the short answer is it depends on how well the user knows how to use Compressor's controls.
FCPX does a good job od down-converting HD to SD, transcoding, and then outputting to a burner or making a disk image. Very convenient if you need to get disk out in a hurry. But if you have Compressor 3, you also have DVD Studio Pro, and used in combination, you have more control over the quality/playability/size tradeoffs…not to mention full-fledged authoring.
FCP does offer one advantage other than convenience – and that is Blu-Ray capability, which wasn't available in the FCS 2 suite.
Yes – in a discrete way.
For example, HD and SD source files will be encoded @ 7.7 Mbps – whether it's 10 minutes or 40 minutes. When hits the play time ceiling at that rate, I recall that it drops to 5.2 Mbps.
Frame controls are on for HD, with the resize filter set to Better; they're off with SD files (since they don't need re-sizing).