At 6 I would expect 20″ and gears even for a small 6 year old.
Islabikes sizing is a good guide to where you’d expect them to be.
My tall 5 year old looks ridiculous on a 16″ now. Mind you she looks even more ridiculous when she plays on the balance bike.
The cheaper 20″ Orbeas look good.
The Islas though win on all points. Weight/resale/brakes/smallest gear ratio. I’ve just looked at every option for the youngest who has demonstrated he can ride and use his sisters. Normal sized 4YO. Mind you he’ll still be getting towed on the 16″ as I’m not attaching the Trailgator to a new one.
A quick ebay scan a 20″ Beinn in the newer colours (not the latest ones) resell for £250 PLUS postage at least. So that’s currently an £80 loss per bike after 2 to 3 years.