Lopez is definitely a big server and he does more than just slicing lefty serves.
As far as who does better against big servers, it's not so cut and dry. "Big servers" do not all play the same. For instance, Berdych plays absolutely nothing like Karlovic, and has a much better baseline game, so the match against him depends on other variables too -- not just how you handle his serve.
Meanwhile, it's different playing Lopez than say Raonic, since that would also depend on how you handle lefties, read their serve, etc...
Most importantly however, I don't think you can just look at Novak and Murray's head to head vs. players who serve big and jump to a conclusion, because said H2H might not reflect who handles big servers better, but who's the better player, or at least, the player who's had the better, more consistent results.
For example, without bothering to dig up data, I'm sure you'll find that Nadal has a better record against these selected players than both Murray and Djokovic, but I don't think that means he handles big servers better, nor does he return as well as those guys.
If the question is simply about who has the superior return, I'd go for Novak, especially once we lump first and second serve returns together. However, he goes through occasional patches where he doesn't get a good read on his opponent's serve, and his returning suffers as a result (it's happened a couple times against Isner. Or even someone who's not a "big" server -- though he serves great -- Roger Federer). Conversely, once he actually gets a read on his opponent's serve, he can go through a ridiculous patch without conceding any free points.
Again though, other factors are involved. Given that big servers are typically big men without great movement, Murray can be better at exploiting the rest of their games (ie frustrating them with defense, counter punching, using a lot of slices, etc...). And yet, even that has a trade-off, since Andy's occasional tentativeness means that if one of these guys catches fire, he may not have an answer, whereas Novak is more comfortable at upping his own aggression and take the match to them.
