Should I buy a used 2x8GB DDR4-2400 CL15 kit over my current DDR4-2666 CL19 kit?

Yes, go with the used 2x8GB 2400 CL15 kit. It’s actually faster in latency — 2400 CL15 has a true latency of 12.5 nanoseconds versus 14.25 nanoseconds for 2666 CL19. You’ll get slightly better real-world performance and double your capacity to 16GB. Just don’t mix the kits; use the 2x8GB sticks alone.