It's so easy for us women to answer this question but it would be so difficult for a man to answer this.
On one side,there's a woman who gave birth to u and brought u up,gave u lot of love and care,etc.On the other side there's a woman who left her home,her parents and siblings and is staying with u as ur wife and expects u to give her love,respect and happiness.
It's so easy to say that a man should equal priority to both,mother as well as wife but nowadays it seems like it's difficult(but not impossible) to do so.If the MIL and the DIL get along with each other then everything's fine.But if there's even a minor problem between the two then it becomes a problem for the man because he always has this fear that if he supports/takes the side of one,then the other will feel hurt.It's like a dharam sankat for the man.Mother will feel son has become joru ka gulam and wife will feel her husband is mama's boy.And in these sitations,it's difficult to remain neutral too.
The best solution I feel is not to interfere in any matter between the ladies.Both the women are matured adults.The man should let the women solve their problems maturely and calmly.This is my opinion.