XPS vs Alienware

These are both outgoing models, if that's a consideration - they've already been replaced.

1.  Unknown, other than that the soldered-in, non upgradeable Alienware RAM may have been forced by the 2080 GPU and slim chassis.

2. The 2080GPU is high-end;  the 1650 is decidedly mid-range for gaming.

3. You won't be using either on your lap, that's for sure.  Both model lines run very hot.

4.  RAID 0 just isn't that much of a performance booster - not worth the sacrifice in reliability in most cases.

You likely would lose a lot of the alienware functionality (which requires drivers that don't exist for Linux.

At the level where you're looking, the newer XPS 15 9500 series (which supposedly has a far superior cooling system to the one you're considering) may be a good compromise.

@riksaga  I don't follow Alienware especially closely, but I've seen multiple threads here with people reporting significant problems getting multiple Alienware system models working properly on Linux, and the response from Dell/Alienware Support was "No Alienware systems have ever officially supported Linux."  So if Linux is important to you, I would strongly suggest that you do some research into what you should expect.  There are only a small number of systems where Dell provides official Linux support, but even among unsupported systems, some will handle Linux better than others.

But in terms of the XPS, you should understand that it is not really designed for gaming, and it really isn't designed for any workload that will keep the CPU and GPU under sustained heavy load.  The XPS systems are designed to be performance ultrabooks, meaning they cram a lot of horsepower into a thin and light chassis.  But that thin and light chassis means they don't have a cooling system capable of running those relatively powerful components at max load for sustained periods of time.  Instead, the cooling system won't be able to keep up, which means temperatures will rise, at which point the CPU and GPU will reduce their performance until temperatures fall to a certain point, then they'll run at full speed until temperatures rise too high again, and so on.

You can't use RAID mode on Linux with pretty much any laptop because Linux doesn't support the Intel Rapid Storage controller that the vast majority of laptops on the market use for providing RAID functionality.  But RAID 0 might also not give you much of a performance boost anyway.  It depends whether the system has dedicated PCIe x4 interfaces for each M.2 slot wired all the way back to the CPU, or whether each slot has a PCIe x4 interface wired to it but the two of them share a single PCIe x4 connection back to the CPU.  In the latter case, then you'll only get PCIe x4 performance across both drives -- and considering that modern NVMe SSDs can saturate a PCIe x4 link singlehandedly, you won't gain much by adding another one.  About the only benefit you might see is the ability to write larger amounts of data at high speed because you'll have effectively doubled the size of your write cache by having two SSDs in RAID 0, so it will take longer to fill that up before the write speeds of the SSDs drop since they'll have filled up their write cache and will then need to write directly to slower flash memory at that point.