I love the mittens sold by lucyblueknits.com. A small family owned business that makes cute and warm mittens out of wool and cashmere sweaters they buy at thrift stores. Each pair is one of a kind. I bought 7 pairs last Christmas as gifts for the women on my list.
I can’t wear wool…so I’m always looking for warm outerwear that does not have wool at all.
But some of these wool suggestions would be great for those who can wear wool.
For OP, the reason that mittens are recommended over gloves for warmth is the structure of the “mitt” - with gloves, each finger is separate and the space between fingers - both on the human hand and on the glove - create air space and thus doesn’t keep hands as warm. With a mitten your hands are more “cupped” - no space in fabric or human hand so the closeness of the fingers “cupped” keeps them warmer.
I’ll add that while I want mittens to be warm, I really can’t stand for them to be too bulky. I feel like a snowman or the little kid who has giant mittens on. 
I personally like a lighter down mitten. And to be honest, sometimes I will layer on my hand - a lighter glove (thinner) under a mitten that isn’t too bulky. And I make the glove have the “touch” feature so if I need to use my phone I can take the mitten off but still have my hand protected by the “touch” glove!
There are many brands of very good gloves and mittens. Many shops in New England will have a good choice of gloves. I generally pick them up at a local store where I can look at them and try them on for fit.
When the weather gets cold (14 degrees counts for someone from California), I go with mittens. Keeping four fingers together helps keep them warm. Also, I wear silk gloves under the mittens. You can buy fine silk gloves that are intended for this purpose. I think that I got mine at Eddie Bauer although it might have been LL Bean. I have had them for years since I only need them on very cold days.