Among many monogamous species, from cockatiels to cichlid fish, studies have revealed a clear pattern: it helps to be more similar to your mate. When mating pairs are behaviourally similar, their reproductive success tends to be higher.
In human terms, this would imply it’s better to be similar to your partner. However, more important than how similar you are – is how much you each come to develop a sense of a shared identity.
Researchers have shown that similarity in couples tends to be more about shared values and background, such as their social class and religion. And these are the factors that appear to predict relationship success—couples with more similar attitudes, values, and backgrounds tend to experience more lasting satisfaction, companionship, intimacy, and love and are less likely to break up.
Here I found a brief write up about which one of it holds true. — Birds of the same feather flock together or do opposites attract?