If that's pants inseam I'd probably lean towards the 21". If that's cycling inseam I'd go 18" for sure. I have a 32.5" cycling inseam and olde horizontal top tube 21" MTBs are generally pretty tall for me in the standover department with stock tires.

I mainly pay attention to top tube length when bike shopping personally. I've had a 15" 80s MTB with a TT just under 50cm and I've a 15" 90s MTB with a 56cm TT. Huge difference there.