If you have got games with FMV, then the lower the better. For the saturn, the maximum recomended burn speed is 4x, However from memory i think that the sega cd's maximum recomended burn speed is 2x.
I found this information out by looking at the covers of official sega CDR media, i have a scan of a saturn one, but i dont have one for sega cd.
I do alot of my games at 8x, but from experience (sorry mal🙂), that stuffs up games with FMV as well as VCD's i burn for my saturn.