If you're going to be in the mountains, I'd err on the side of warmth.

I was touring there in August and in the early mornings (say between 6:30 and 9am) and evenings (after 8-9pm) it was rather cool. Many mornings I started with armwarmers and sometimes even a vest. Evenings I was always wearing long pants and a long-sleeve something plus often a jacket. If you do not tend to feel cold like me, you might not need so much.

Lucky you, it's going to be beautiful!!