Which is worse?

Both aren't good. A matrimony should always be between two people. Three is a crowd! There will always be regrets, pain and frustration at some point in time with more than 2 people involved.

Yes, I quite agree with you on that, a matrimony should simply be between a man and a woman. That said, however, there are places in the world where it is allowed for a man to marry more than one wife especially if he is capable of caring for them, and they have no problem with that. Now, which is more justifiable marrying more than one wife with no concubine or marrying one wife with concubine(s) or girlfriend(s).