There are 19 World Tour teams. 11 of those (Ag2r, Astana, Bahrain, Bora, Cofidis, DQS, EF, FDJ, Intermarché, Lotto and DSM) have not got a single rider who has won a GT. 6 of those (Ag2r, Astana, Cofidis, DQS, Intermarché, Lotto) have nobody who's ever made a GT podium. And some of those teams I've eliminated there only have GT wins or podiums from riders who are unlikely to scale those heights again - Qhubeka only have Aru who fulfils the criteria, while Trek have the aging Nibali (not much different to Valverde) and Bauke Mollema whose podium was the 2011 Vuelta, acquired eight years after the fact with Cobo's removal. Jury's still out on ISN, because Froome doesn't look like recapturing his GT winning form but if there's anybody that is going to turn it around miraculously it's him - however while Dan Martin hasn't got a podium, his best GT result is his 4th in last year's Vuelta.
And, as I mentioned before, the other thing López has over Mas as a leader is that he's a proven winner. Even if a GC bid goes way awry, Supermán can do something to salvage the race in the same way as Quintana at Col de Portet or Valloire, or Valverde at Peyragudes, which Mas hasn't shown the ability to do to date. And López signed off the back of winning the queen stage of the Tour de France. In terms of final results on the GC at the end of the day, Mas and López might be much of a muchness, but if I'm a sponsor, I'd much rather have Supermán - he's going to be a lot more visible throughout the race, and he gives a much greater chance of seeing my company logo all over the media.