[riskable]

Not much of a difference, actually. Once you've got your brain trained to select the letters by location you can remove the eye tracking part and just use abstract thought.

"I'm thinking of a letter... On the far left of the keyboard, middle row."

[bonoboTP]

I think there's not enough bandwidth for such information density. These usually work with 2D directional commands, you're driving the cursor up/down/left/right. You're not thinking of "letter E", but look at the screen and think of the direction where the cursor has to move from its current location to drift to E.