This is a hard question to answer in a vacuum.
Mantha is the better pure "Top-6 Scorer" option on skill. But he's one of those players where if i'm shopping for him as cornerstone Top-6 play driver, my team is probably not very good, and i have some serious holes. I want him as my "complementary scorer" option to fill things out.
Whereas Anderson is much more the "Complementary Middle-6 Scorer" profile, where i think you have to expect him to be about as good as the more skilled and creative linemates feeding him and generally doesn't have quite the same raw skill level as Mantha. But he brings so much else to the table, it's hard to even compare the two straight across, because they fill a totally different role.
But that's where the conundrum applies for me. They're both fundamentally "complementary goal-scoring wingers". On completely opposite ends of the spectrum. Neither is really going to carry or drive a line. So in that respect...i'd rather have the situation where Josh Anderson is the player i'd prefer - needing to add size and power to my Top-6 scoring line. As opposed to the scenario where i'd prefer Mantha - trying to eek out an extra ounce of skill to my Top-6 scoring line, even though he's really not a line-driving player anyway.
So...I want Mantha on my team. But even more than that...i want my team to be in the situation where i want Anderson instead.
Essentially...for me valuing the two, whether i'd rather add a Mantha or an Anderson, is probably more truly a reflection of how my team is constructed, than a reflection on either in a vacuum.