I'd go with a beefy desktop, either an iMac or a beefier Linux box (I'd not bother to assemble one from parts - would probably order it from Dell, Lenovo or HP's line of workstations or smaller servers), and a thin MacBook/MacBook Pro for portability (because Macs are unbeatable laptops).
I also have a smaller laptop, for going to hackathons, an Acer "Cloudbook" that came with Windows and now runs Fedora. Very small, light, so-so in performance and cheap enough I don't care too much if I need to replace it.
With the question posed that way, my top choice would be to wait for the 2018 MacBook Pros. Or if it works for you, an Air or simply a MacBook.
But maybe you mean non-Mac alternatives.
Purism is intriguing. I don't have personal experience with their stuff though.
If I really didn't want a MBP, I'd probably get a MacBook or a Chromebook and then lean on a Linux desktop system for heavy duty number crunching type stuff. That's similar to what I do now except with an older MBP.
I also have a smaller laptop, for going to hackathons, an Acer "Cloudbook" that came with Windows and now runs Fedora. Very small, light, so-so in performance and cheap enough I don't care too much if I need to replace it.