In my opinion since Arduino targets the hobby sector (and education to a certain extent), the main points are both the tutorials the and community support.
The longer a board survives, the richer the experience and on-line references.
I kove Nano because is simple and there are piles of info on the net. But I agree that a bit more memory and a bit more processing power will be nice. More support to I2C and shift to 3.3 V as universal may also help, in my view.
From my experience, however, none of the Arduino products are suitable for education. They lack the pedagogy perspective.
I am working right now to "develop" a suitable Arduino board for kids, based on nano. 