I would argue that the most important theme of this work is actually that of love and how the love that exists within a family can be so important and vital in the development of character. I do think that both of the themes you ask about are perfectly valid themes that could be analysed, but the theme of love is the most prevalent.
If you think about it, there are lots of different kinds of love represented in this work. There is the power of parental love expressed through the parents of Tish. Sharon Rivers, for example, shows an abiding concern not just for her daughter but also for her unborn grandchild when she discovers that Tish is pregnant. Tish's father is likewise very supportive of his daughter, even though she is not married. Baldwin seems to suggest in this novel that human love is what makes the world literally go round. What sustains the family through its various trials and joys is love, and love keeps them functioning and cooperating together. Love is also shown to be the necessary force that is needed to build a world that will help the children grow up safe and secure. This work also shows us through other characters the impact of growing up without love and the kind of lives that this results in.
Therefore, I think that the two themes you suggest are very strongly represented in the text, and in a sense, identity formation is a corollary of the theme of love, but I would argue that love is the central theme of this work.