I’m 5’11” and ride a medium hardtail (18 inch) and a large short travel susser (19.5″, I think, but I never measured it). I got fairly long legs (33″) and both fit me well.
I find the hardtail terrific on the easier stuff and climbing, but my weight’s too far forward for any bonkers downhill – the susser, with it’s longer top tube, it much much more stable. I can barrel through stuff on the bigger bike that has me sh!tting myself on the smaller bike.