If you want to do the shortest way to GRU to avoid 17 hours in Mexico (you may run out of your test hours) definitely go for 6:15am departure from BCN and day flight on Air France 777. I have had max 10 people in J, next week is only 5 of us as of now. If you want to do layover in Mex on the way to Gru, I'd go for AM for a bit better seat, quick and good food and better chance to sleep all flight long.

Way back I would jump on KLM, so far these flights were best long hauls on this itinerary and avoid MEX-MAD, the flights are full.

This was my experience in February REPORT, last week we came back and it was on AF777 and KL787, a bit better experience, even AF service was not as good as you'd want but hey we were off to GRU the very same night without long layover in MEX.

Edit: I wrote a small "report" also on this itinerary about AF 777, page 37 flyer talk af