Precision T5810 bios set up should I choose ATA or AHCI I don't need Raid

Precision T 5810 Win 7.1 64 bit two hard drives.

I just got this PC, and looking through set p, I see that RAID is enabled. During boot up I get a screen offering options for the RAID controller, but I don't want to use RAID, so getting that screen is unnecessary, just increases the time to get to Windows.

The two hard drives are both  ATA  ST500DM002-1BD14 500GB, one hard drive is for the OS and installed software (plus also Dell's recovery partition), the other is for my documents and other data files.

Apart from RAID (apparently the default enabled option), set up offers ATA or AHCI. Which should I choose?