In my opinion, having more keys isn't necessarily better. Being able to easily access those keys is more important. I know for many people, they like to be able to look at the keyboard and see a key and just hit that key. For me, I like the ability to layer additional functionality on the existing keys. I use my thumbs to momentarily hold the layer (think shift key, but instead of Caps, doing other things like symbols, or macros). But I touch-type, so I hold an image in my head of where all the keys are and I just hit them, with or without various modifiers. I know this isn't a solution for everyone, thus there are many different styles of keyboards, and many different ways to layout those keyboards.
I think this is the reason I like the current ErgoDox layout. I have a couple big 2U, easily (for me) to hit and hold keys for my thumbs. This allows me to use the other 4 fingers on the same hand (or opposite hand) to move up and down the rows to hit all the other keys. All the smaller 1U keys in the thumb cluster don't bother me as much. I end up moving the opposite hand to hit them with my index fingers, or map functionality that is significantly less used.
So, for me, having a bunch of thumb keys, and the ability to modify the keyboard firmware completely are the most important parts to increase efficiency.