This is presumably just the full board versus half nomenclature noted above. But yes, consumer GPUs are way more cost competitive than Tesla class parts. Being able to train bigger models is valuable to some folks, but not everyone, so I don't begrudge using the GTX line.
(Also, I can no longer edit, but a colleague pointed out that I should have read more carefully. A GTX 1080 is a Pascal part, which compared to the poor old Kepler in K80s, it'll really shine. Volta all the more so in the next year).
Disclosure: I work on Google Cloud.