Assuming you've got an Intel ICH, there's really no good reason to run in AHCI mode if your system supports RAID mode. Your performance will be *identical* in RAID vs. AHCI mode if you're using a standalone disk. However, if you install into RAID mode, you have the option of installing a RAID volume at a future point in time without having to re-install Windows-- even if you're not using RAID at all right now. It's essentially a matter of future proofing.
The only reason I can fathom for going AHCI rather than RAID mode is if you plan on migrating your system someday to an ICH-based motherboard that doesn't have RAID support. Possible, but it seems more likely that you'd want to experiment with RAID someday with the system.