Alienware. Take minimum RAM/SSD et replace those by yourself (you can do this on the 17'' version, double-check that this is also true for the 15'' version. I think I remember some crap with the x15 like the RAM being soldered). You get the Dell on-site guarantee, and the machine is much cooler probably in both senses.

The real issue with both machines is that your GPU is soldered to the motherboard and thus will likely kill your laptop eventually.

Also, good to know before you buy, I got myself an AW x17 R2 for prototyping and gaming, and I realized that the built-in speakers make the chassis vibrate and create a terrible crackling noise if you use them at mid to high volume. This defect seems to be present on the whole series. Also, the webcam is crap, and the battery doesn't last long. Not sure if the Lambda laptop is any better in these regards, though.

A better bet might be the MSI Raider GE76 (if they have a 15 inches equivalent), but it looks a bit more flashy / less professional, you don't get on-site repairs, and the power supply is less transportable I think.