This is an interesting debate, but I think the bottom line is that Movistar needed another Top 5 GC rider.
I rank Lopez, who is one of my favorite riders, equal to Mas, in the sense that both seem to be around a 5th place GT rider -- but Mas will get there in one way, and MAL in another way.
Talent wise, I think MAL is better but that's not the whole equation, obviously. Mas quietly had a great season this year and may have more headroom than I thought when he was on QS. Either way if you're a Movi fan this is great news. If each rides to his potential, the team could have two podium finishes next year.
Mas is like a new Carlos Sastre. Sastre had an incredible career and of course it was capped with that Tour win; he scored 15 Grand Tour top 10s and 6 podiums; however, he was not explosive and was best at riding himself into form as the race went on, meaning he didn't amass much of a palmarès in short stage races (plus he raced through the super-peaking era), and only won five stages across the totality of that Grand Tour lifespan, and he became the poster boy for letting go early and riding the climb at his own pace, frequently being dropped early on the climb only to re-emerge riding through everybody and making his way back to the front as they all blew up, like Pello Bilbao in the Stelvio stage. Mas is a great GC rider, but without more explosivity he will be a guy who scores a lot of placements but won't score so many wins.
López' win column is already enough that many riders could retire on.
I had a couple of comparables from cycling history of the difference between the two, but they didn't truly work. The two I was looking at were Sastre and his brother in law (I think the Sastre comparison for Mas has mileage), and also mid-2000s Euskaltel, with Mas as pre-ninja Zubeldia, and López as Mayo. The reason both of these comparisons fail is partly to do with the eras they are racing in (I'm not ready to anoint Supermán a greater climber than Jiménez, though his palmarès is already pretty much there - Chava has more GT stages, López has more results outside of them and better GC results overall), but also largely because, put simply, López has delivered far more on his promise than Mayo, and Chava's personal demons derailed him often enough, even in the middle of his heyday.