There's no single correct answer. It depends on the size of the file you are trying to burn to disc, and also the amount of space available on that disc.
SP is a fixed data rate.
Flexible Rate will vary the data rate (i.e. quality quality) to suit the amount of stuff you want to record to disc.
If you have say a 30 minute program and a completely blank disc then FR will (or should) burn it at the highest possible bit rate for the best picture quality.
If you have 2 hours worth to fit on the tail end of a part recorded multi-session disc the FR will drop the bit rate through the floor. The picture could end up a lot worse than VHS, but at least the recording should squeeze on.
SP doesn't do any of that. It records at one rate. If there's space left over, or the recording over runs the disc space then tough. But you'll know exactly what the recording rate will be and can make that judgement before you commit to burning.
