Get an 18", top tube length difference will be small, and a 20" will look like a gate. 🙂
I had a 16" at 5'10" with long arms, with 65mm stem and no layback post but the seat back in the rails a bit. Was a bit wheelie prone when climbing, hence the longer stem on my new slotty inbred.
You have to spec them pretty bling to get the weight much below 30lb though, so don't expect it to be a rocketship up hills unless you've got deep pockets (or suspect scales :D). Quick downhill with 140mm pikes on though.
I got rid of mine as most of my riding is in the Purbecks with plenty of climbing, and the trade off for 5 minutes fun downhill was the hour and a half climbing each ride to get up there in the first place.