I'd warn against working on obscure operating systems and kernels unless you were independently wealthy so that this could truly be a hobby. You may invest years of your life into an OS, only to find the job market virtually non-existent. It's possible to switch kernels later in your career (and having worked on another kernel provides valuable perspective), but good luck convincing employers of that (some get it, but they are harder to find).
There are plenty of jobs for Linux and FreeBSD, and I find both kernels very interesting. As for architectural interest, I can see a case being made for Plan 9 or unikernels, but most other kernels are pretty similar.
I don't see the point here. The majority of OS hackers are hobbyists to begin with, and learning should be the top priority in any event. Learning for the sake of being employed as a kernel hacker sounds backwards and an unhealthy approach. You demonstrate your skills and then get hired or apply for a job after the fact.
Besides, it's not like you can't be a Linux sysadmin/programmer in your day job and hack on more novel OS as your hobby.
For example, Al Viro was an ex-Plan 9 hacker who later became a full-time Linux guru.
The thing is that once you've been involved in truly different OS architectures, migrating to a relatively boring Unix monolith like Linux isn't all that hard. Besides, Unix and Linux are so widespread there's little chance you won't be using them in some form throughout.
As for architectural interest, I can see a case being made for Plan 9 or unikernels, but most other kernels are pretty similar.
Sorry, I should have been more clear: your point was about architectural differences. I find unikernels and distributed operating systems like plan 9 very architecturally different (for reasons I'm sure you know). I'd probably include microkernels like Minix 3 as well.
But monolithic kernels, like Linux, BSD, and Solaris, being architecturally different? I think if your goal was to explore different kernel architecture, you'd be better off looking at the others, as these are relatively similar. I think you'd even agree!
There are plenty of jobs for Linux and FreeBSD, and I find both kernels very interesting. As for architectural interest, I can see a case being made for Plan 9 or unikernels, but most other kernels are pretty similar.