Costs less and appears to work just as well.
Note - 16MP without a bayer arrangement or filters or other crap nets you 4080*4080(16MP) or roughly equivalent to ~25MP in color. It's more than adequate. The 39MP model is 7216 x 5412, but has smaller pixels. Still, while it's a bit less sensitive, that's... (calculates) roughly 60-65MP to get that quality with a color sensor.(just factoring in the Bayer interpolation losses and not any filtering or other junk that gets in the way)
The smart money though seems to be on the square format 16MP. 4000*4000 good clean pixels looks to be just about right.(though it's still 10K+ for the back... sigh)
NOTE: Foveon could make a killing with their sensor here. I keep hoping they'll make a 4000*4000 sensor some day...