Better in what way?
Good for whom?
Which cultural context?
I think it would be possible to set up studies to compare monogamy vs. poly for some particular parameter that was accepted as good/better: say - asking people how happy they are with their relationship structure on a ten point scale - and then comparing the two averages. But then, to be truly rigorous, you would have to get all of the participants in the study to switch to the other relationship structure and rate their happiness there (perhaps people who choose poly relationships are happier - or less happy - in ANY relationship).
So much depends on the individuals involved and what is better for them. Even if you did the "happiness study" above and showed that, on average, one was "better" than the other - that doesn't say anything about what is better for individual people in specific circumstances.
Which is why I said that they "shouldn't" be compared that way - not that they "can't".
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (23+ yrs)
Dude: hetero poly male, live-in boyfriend (4+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe