Short version: If you're about to clean install an OS, switch to AHCI. RAID mode offers no benefit on an XPS 13 that only supports a single SSD. If you're curious about why it's there, read on.
Long version:
RAID mode seems to be the default on most if not all Dell laptops and desktops that support it, except for the handful of systems that Dell offers with Linux pre-installed from the factory. I suspect Dell does it these days simply to standardize their builds a bit and also because it doesn't have any downsides for them, but it can have some downsides for users.
RAID mode activates the Intel Rapid Storage controller, which abstracts the storage from the OS and allows certain other features to be used. Back in the Windows 7 days, that abstraction meant that RAID mode could be used to allow Windows 7 to be installed onto NVMe SSDs. Windows 7 didn't have native support for NVMe, but with RAID mode, the OS just needs the Intel Rapid Storage driver and then it doesn't matter to the OS that the storage "behind" that controller is NVMe. By comparison, AHCI mode exposes the storage directly to the OS, which means the OS needs to have native support for the storage device's data interface, i.e. NVMe in this case.
But RAID mode is also required for using certain other features, such as Intel Rapid Start, Intel Smart Response, and more recently Intel Optane. But the first two are only used when you're pairing a spinning hard drive with a small SSD cache, and Optane is only used with actual Optane devices.
In terms of downsides:
- Depending on the generation of Rapid Storage controller in your system and the version of Windows you're installing, RAID mode means that you might need to supply the Intel RST driver during Windows Setup to allow it to see your SSD. Not a big deal for Dell since it's just one more driver they'd have to inject during their factory setup process.
- RAID mode prevents you from using Linux with the internal disk since Linux doesn't seem to have an Intel RST driver. Not an issue for Dell in most cases since they sell very few systems with Linux as a pre-installation option.
- If you buy a retail SSD from a vendor that offers its own NVMe driver, such as Samsung's retail SSDs, then you can't use that driver if your system is in RAID mode. Not an issue for Dell since the SSDs they sell don't allow using those drivers. Even if you get a Samsung SSD shipped with your system, Samsung's NVMe driver won't work with it. You need to use a retail unit.
So again, for Dell I guess it makes sense to just use RAID mode everywhere for consistency, since they of course do sell some systems with Optane (and Smart Response and Rapid Start in the past), as well as other systems that actually do have multiple disks and therefore support actual RAID setups. And the downsides don't really matter to them.
But for individual users performing a clean install, switching to AHCI means you don't have to worry about providing an Intel RST driver, you can use Linux if desired, and you can use a vendor-provided NVMe driver if desired (and available).
However, this setting is only really meant to be changed before reinstalling an OS. If you want to switch WITHOUT doing that, you'll render your OS unbootable until you switch back. Apparently it's possible to work around this by booting into Safe Mode ONCE after making the switch, which will allow Windows to start and reconfigure itself. After that, you should be able to boot normally. But if you don't need any of the benefits of AHCI mode, you're not really losing anything by sticking with RAID.
