Hey there, many times I had similar dilemma :- ) The best answer really changes through years.`
(I try to be impartial in my answer below, but I want to note that I receive few free laptops from Intel here and there. But I built plenty of other computers myself, from SFFPCs to big server-builds, so I can compare stuff from experience.)
5 years ago I think, laptops were really weak (although there were few desktop Ryzen in ECO mode (65W) put into few Clevo chassis), so I built portable SFFPC with Cerberus-X and put 18-core i9 into it, was still bit weaker than my 32-core Threadripper at that time, but so much more powerful than any laptop on market. This was quite heavy SFFPC and it wasn't easy to lug around. The really small SFFPC were impractical in shape and only supported ITX boards, which mean 2x32GB ram limit, too little. Nowadays, 2x48GB ram is plenty for ITX boards and mainstream laptops since 96 is quite bit closer to 128 than 64GB was.
2 years ago, Intel came with HX chips, effectively desktop CPUs in laptop. Shortly after, AMD came with their own HX, which is lot more energy efficient :- ). But there are few laptops with it only, mainly Lenovo Legion series. I have 12950HX Dell Precision 16, 13950HX MSI Titan GT77, and will probably get Razer Blade 18 14950HX at some point.
My observations:
- These laptops are really powerful, the CPU chips themselves are absolutely identical to desktops. But that depends on chassis and how much they can supply power and cooling. For example, the Dell Precision is rather thick and heavy workstation (not the XPS-body style, the big-body version), but has rather weak cooling, so it's like 40perc. weaker than my MSI Titan.
- They are very loud if you set them to desktop performance. As in, people will be angry at you in office :- ). You can put on noise-cancelling headphones on. But you can just keep swapping between performance profiles and keep the laptop reasonably silent for most work, and set it on maximum only when you need it.
My MSI Titan, is big, ugly, plastic and loud. But it's otherwise absolutely identical in performance to 13900K, or 7950X, etc.. Engineering marvel. So the only time it would make sense for me to build SFFPC, is to build mini-Threadripper. Because mainstream CPUs don't outperform HX-series in big-body laptops like the Titan.
Medium-body laptops like Razer Blade are slighly more compromise, but they are more portable and easier on eyes (nice metal chassis, clean design, better display calibration from factory,..).
Unfortunately, all laptops of this sort are now like 4000-5000 Euro if you also want high-vram GPU before you even upgrade ram to 2x48GB.
Asus Strix and Lenovo Legion are most reasonably priced and have best performance/price value. Lenovo has better quality control. MSI and Razer are really expensive, with MSI Titan-series you get desktop performance (it's like the only laptop with 350W power delivery, insane thing), with Razer you get the only Macbook style body.
Laptops are imho much better than SFFPCs since you also get a display with them, so the moment you plug into external 27" monitor in office or elsewhere, you get free side monitor on the laptop itself.
SFFPC really only make sense if you want to maximize GPU performance, so they're really more like niche gaming builds. If you follow the Reddit/r/sffpc, then you know the audience :- ). 90perc. of them are super impractical and barely cost effective against laptop and unless they have 280mm AIO on CPU, the CPU performance is often just as laptop, so it's just the GPU working as intended.
