Because of the combined_mode issue with the piix/ata_piix drivers, where both drivers try to control the chipset (resulting in no DMA and incredibly slow performance), it's better to have only one driver or the other, rather than both.
(Yeah, I'm complaining about a bug that affects me... Very self-centered. This is a common chipset though, so I'm probably not the only one getting this issue.)
For my two cents, I'd suggest sticking with the IDE driver for now - nobody using TC is going to need the features of the SATA driver, which are more useful on servers and the like.