As this was not explained:
Reasons why you want AHCI:
-> NCQ support (increase HDD performance)
-> TRIM support (prevents the SSD performance reduce over time) (Win7 and up)
-> eSATA support
-> Hot swappable feature
-> Full SATA speed (not really, it depends on the controller. Some don't speed limit itself)
Only use IDE for legacy OS such as Windows XP. Modern OS, such as Vista and newer, use AHCI (or RAID if you have one)