I am solidly in the camp of the internal hubs at this point especially on the smaller wheeled bikes. On the 20" wheels my choice would be the Nexus 8 spd premium hub I have on a few bikes as it is very efficient and it shifts better than any other hub under load. Shifter to lower gear is amazingly quick and easy. I love being able to shift when stopped. I have a good gear range too.

For 16" wheels the hands down favorite for me is the Sturmey Archer 8 spd. It the best usable gear spread using normal sprockets of any internal hub on this wheel. It 1st gear is a 1:1 ratio so I don't have to use huge or tiny sprockets to get the gear range I need.

Efficiency on the latest internal hubs is so close to RD as to probably be an almost moot point. One BIG plus for me is no rear derailleur hanging down being a dirt magnet. Riding on the beach and on unpaved trails with my RD setup collected dirt, sand, twigs, etc.

I only have one folder left with a rear derailleur and it will soon be gone.