I’m thinking Melanie wants each of them for different reasons and has different emotions.
Phillip – He’s rich, exciting and everything she’s always wanted in a guy. He’s a bad boy in his own way and comes from a family as disfunctional if not more so than the one she grew up with. Even though Victor looks down his nose at her she feels she fits in.
Nathan – He’s stable, comes from a good family and has Maggie for a grandma. Everything she’s always missed and whether she knows it or not it’s what she’s always needed. She doesn’t feel she deserves Nathan, that she can’t measure up and she’ll be an embarassment to him and his family
I can see her having a stronger relationship with Nathan once she gets over her insecurities and lets herself be loved like she should be.