Here's what I see when Googling those hardware options and the links to Dell:
alienware 17 R5: laptop, only 2 variations; 8th Generation Intel® Core™ i7-8750H, Windows 10 Home 64bit English, NVIDIA® GeForce® GTX 1070 8GB GDDR5 (OC Ready), 16GB, DDR4, 2400MHz; up to 32GB, 512GB PCIe M.2 SSD Class 40 + 1TB 7200RPM HDD
alienware M17: laptop, various configurations up to; 8th Generation Intel® Core™ i7-8750H
Windows 10 Home 64bit, NVIDIA® GeForce RTX™ 2080 8GB GDDR6 with Max-Q Design, 16GB, 2x8GB, DDR4, 2666MHz, 512GB PCIe M.2 SSD + 1TB (+8GB SSHD) Hybrid Drive
alienware area 51: desktop, various configurations, up to: Intel® Core™ i9 7960X, Windows 10 Home 64bit English, NVIDIA® GeForce® RTX™ 2080 Ti 11GB GDDR6 (OC Ready), 64GB Quad Channel DDR4 at 2666MHz, 1TB M.2 PCIe SSD (Boot) + 2TB 7200RPM SATA 6Gb/s (Storage)
In our article on the graphics card for that level of complexity we talk about a GTX 1060, however, that is the absolute minimum. so a GTX 1070/RTX 2070 is that much better, but mainly in rendering speed.
You generally should always err on the cautious side and then any benefits of having more horsepower from the hardware for lessor Scene
requirements, the better.
The performance gain you get from the Ti series when rendering is quite significant. But everything of course also depends on your budget.
So for a requirement to do ‘very complex scenes’, then I would suggest at least a GTX 1080/RTX 2080, and preferably a GTX 1080 Ti/RTX 2080 Ti. And with at least 32GB of RAM.
Also see our Knowledge Base article: Which laptop PCs do you recommend? and our Blog Lumion Hardware 101: Minimum and recommended requirements for faster 3D rendering performance.
If you are now considering desktops, then that opens up a whole new set of options for you. So the decision first needs to center around whether mobility/luggable mobility is more important than horsepower. The benefit from a desktop is generally better performance and flexibility, plus they also tend to have more performance per $ spent. If you find a comparable laptop, that is, one that is a full GTX/RTX graphics card and not the Max-Q or lesser configuration, then the two options are much the same in terms of performance.
Never an easy decision to work through with the options available. But, again, see our articles and Blog, and in summary my suggestion above.
I hope that helps.