IDE is a standard mode of communication between a hard drive and a SATA-enabled motherboard, whereas AHCI is a newer, faster and more customizable mode of communication between hard drives and motherboards. When compatibility is not an issue, AHCI is the better of the two options because it gives better read and write times and more extensive design support than the older IDE does.
AHCI offers advantages such as native command queuing and hot plugging, which are not available in IDE. The disadvantage of AHCI is that it is only supported on newer hardware and operating systems. Windows Vista and above, Linux, OpenBSD, and Solaris 10 all support this new standard. Meanwhile, all operating systems from DOS onwards support the older IDE.
AHCI is also capable of emulating other, less common modes of operation, including Legacy and vendor-specific RAID modes, to facilitate hardware and game design. Choosing AHCI over IDE can also improve bandwidth speed in performance solid-state drives. Switching between AHCI and IDE while an operating system is already installed can lead to data storage problems.
Visually inspecting the connectors between the hard drive and the motherboard can help techs determine whether a setup supports AHCI or not. If SATA connectors are used, chances are good that AHCI is supported for this hardware. If parallel connectors are used, the motherboard and hard drive combination probably only supports IDE.