Eneloops all the way. There's a thread where people talk about chargers, but none of them suggest there are any better batteries. I use the 1900 but have used a higher mah before.
"if eneloop 2500 are the..." is the topic
https://ukcaving.com/board/index.php?topic=16158.0
I just plumbed for a higher number of charges before the battery decreases it's capacity. The 1900's suggest their capacity stays up with more charges, whether it matters at all I don't know, but worth a shot.
I've also upgraded my charger, which I think will make the difference. Each battery will be charged separately rather than like the simple wall chargers which charge two on the front and two on the back.
I reasoned this change would help keep capacity up, as pairing a good battery with a low capacity battery could lead to less capacity all round.
Duos use all four batteries together, but at some point one battery will be lower than the rest, if you marry this up with a good battery on charging, then the bad battery will not charge completely. therefore reducing the length of time your light can be ran for. If now you pair up the bad battery with another good battery, then it will still not charge completely.
Whether any of this is true and a bad battery can bring down the whole set, I don't know but I'm experiementing, so we'll see how it goes.