spinlock_t
If the goal is to optimize for $$$ earned over one's career, there is more money in being a back-end/distributed apps developer. There are simply more roles (at the higher levels) and companies working on these problems. The progression for SW engineers is junior->senior->staff->principal->distinguished. As you get closer to staff, there are very few systems roles left. Some people never make it beyond staff/L6/E6. (One could argue that it's ok to stay at staff for the rest of your career and still make good money. But there's always more to be made.) And the ones that do, are the ones that spearhead new products/features (direct business impact). My 2cents, after 15 years as a system dev in the valley.
Behave - Robert Sapolsky.
Gene: An Intimate History - Siddhartha Mukherjee