I have a bit of a problem with this argument because it's only used one way when it should be used both ways.
People always say how Benzema's goal output suffers because he had to adapt to Ronaldo, but the argument can be reversed (although it never is during discussions!) and it can be said that Lewandowski's assists count suffers because he never plays with equal or better finishers than him, so that he could take a back seat and create for them.
When I think about strikers that are ahead of Lewandowski, I think of Romario and Gerd Muller. Strikers who could match his longevity and consistency. Not about van Basten or R9, who had relatively short peak. Also I tend to disregard strikers who had all the skills required but couldn't put them into proper spotlight like Batistuta (despite me being a huge fan of his) or Kane.
To answer the question: I think it's not a controversial statement at all to slightly favor Benzema over Lewa right now, or this season only. You have to admire how he became a true leader of this Madrid squad. Also, this is a WC year and I think it's a safe bet to assume that France will do better than Poland there
so Benzema's stocks will only rise while Lewandowski's not so much.
If we're talking entire career then I believe Lewa is ahead by quite some distance. Maybe Benzema's fans are just really bad at arguing for his cause. If I was to do it, the first argument I would use would be the period of time when Benzema was tearing it up at Lyon while Lewandowski was only starting in the Polish league.
The "but he's better all-around player" argument doesn't work on me because we live in a period of time when almost all offensive players strive to be good all-around and every year we have a batch of new so-called "generational talents" who can supposedly adapt to any position over the offensive four. Meanwhile there's a drought of great goalscorers and maybe that impacts the way I rate players. This creates a situation where it's probably easier for a top club to replace an all-around player than it is to replace a great goalscorer. Maybe it's the matter of supply and demand after all?