I'm 6' with a 34" inseam and currently ride a 61cm Quantum pro and an XL (22") Adroit. I feel this Adroit is just on the verge of being too small, and the previous 21" Adroits I've owned were definitely too small. My first Klein was a 20" Rascal, and I'm still irritated that the shop ordered that size for me without measuring me up.
I wouldn't go with a 20" without trying it out first... unless of course you like a LOT of seatpost hanging out and a LARGE drop from seat to bar.