Razer peripherals tend to be worth the extra money spent. They’re durable and high-quality. However, their full-fledged PCs tend to be marked up and not worth the money.
While Alienware’s PCs do allow for personal expansion, their manufacturing tends to make it a little more difficult to self-upgrade your PC because of unique, non-standard manufacturing practises.
The two companies are too different to make a direct comparison. For example, Alienware makes better PCs and laptops than Razer, but Razer makes better computer peripherals.
Razer primarily manufactures computer peripherals and gaming laptops. In addition, they attempted to manufacture smartphones, but this venture was unsuccessful and has since been discontinued.