I use Thinkpads for Linux as they are well built and trusted brand that has yet to disappoint me and are well supported by Linux. Plenty of room for System76 to just do a Linux-thinkpad better by focusing on the Linux laptop segment of the market.

Edit: I just popped over to System76 to look at their laptops and I must say I really prefer the way Thinkpads look when compared with System76. I realize that this a dumb reason to choose a laptop but I have encountered no better judge of build quality than how a laptop looks. Every stupid looking laptop I've ever bought has been a disaster (parts break off, camera dies, weird crashes), whereas laptops who had a physical design which I liked (macbooks, thinkpads) have held up much better.