Alienware isn't really a fair comparison - their machines aren't what they were, since they've gone towards a mass market in Dell's hands. Alienware stuff used to be extremely high-performance, not always the most stable (and in the case of laptops, often stretching the definition of portable) - about the same price as Apple, but giving up stability and portability for added performance (especially GPU performance). More recent Alienware machines are often essentially Dells.
The best comparison to the MBP 15 is probably HP's or Lenovo's lighter 15" workstation models - fast, well-built thin and light professional notebooks.
I couldn't get Lenovo's configurator to give me anything but cheap 1920x1080 screens (they seem to be out of the decent display options), but I DID get HP's site to price a Zbook Studio that is quite close to the MacBook Pro 15" base model - and it's almost $100 more expensive ($2891), although it includes a 3 year warranty.
Processor - Same (7820 HQ). HP's $350 processor upgrade is a Xeon that has the same base clock than Apple's $200 upgrade to a 7920 HQ, with 100 MHz more turbo.
RAM - Same (16 GB - HP uses DDR4) - note that 32 GB is an available ($350) upgrade on the HP
Storage - essentially the same (512 GB PCIe SSD - HP's Z Turbo drives are at least very close to Apple speeds - sometimes literally the same chips). Either can be upgraded to 1 TB for a reasonable cost, while Apple offers 2 TB for a king's ransom and HP offers DUAL 1 TB SSD's (just as expensive). It MIGHT be possible, although not officially, to get the HP as high as 4 TB if you could find 2 TB SSD cards that fit!
Display - similar (HP is 4K, but probably doesn't quite have Apple's gamut - for $50 more, you can get an HP DreamColor display that is probably closer in gamut)
Discrete Graphics - comparable (Nvidia Quadro M1200 - looking at Notebookcheck's benchmarks, it's slightly slower than the Radeon Pro 560 and a tiny bit faster than the 555). HP does NOT offer faster graphics on this model, although they do on heavier mobile workstations.
Size and weight - Apple is slightly slimmer and 0.5-1 lb lighter (HP claims 4.5 lb, but they always weigh the lowest end model, and this one has quite a few upgrades). HP's power cord is also almost certainly heavier, so the travel weight will differ by close to a pound.
Ports - significant advantage to HP (in my mind). Only 2 Thunderbolt 3, but gain 3 USB 3.0, HDMI, SD reader and even Ethernet (!!!) The power cord does NOT take up a Thunderbolt 3/USB 3.1 port.
Touch pad, keyboard, touch bar - Slight advantage to Apple. The keyboards are comparable (both among the better non-mechanical notebook keyboards), while Apple offers a better touch pad and a touch bar in return for HP's lesser touch pad plus "eraser head" pointer.
The ZBook 15 Studio is almost exactly the PowerBook many of us are dreaming of (with the possible exception of really fast graphics) - it's in the weight range we want, it takes 32 GB of RAM, has both modern and legacy ports, and it has a screen that is AT LEAST the equal of Apple's.
If the choice were hardware only, I'd take the HP over the MacBook Pro - both are beautifully built, and HP's choices meet my needs better than Apple's. The problem is that Apple won't either build a machine like the HP OR license High Sierra to HP. Picking the HP means accepting Windows and losing system-level color management, some amount of stability (Windows is getting better, but it's no Mac) and Apple's ecosystem integration