Neither. Circumstances put them at odds with each other.
There are subtle differences to the way Homura interacts with the non-Madokas. For example, Homura can rightfully state she doesn't give a damn about Sayaka because everything that happens to her is due to her own actions and complete inability to comprehend the gravity of the situation. While Homura doesn't actively wish ill will toward Sayaka, she views her as a major thorn in her side.
In Mami's case, she respects, pities, and fears Mami. Homura can't do anything to save Mami because Mami is a veteran magical girl. Telling her the truth shatters her view of the world and it leads to people dying. At the same time, Mami is Homura's mentor and we see in E3 that Homura does feel saddened by Mami's death and in Rebellion where she states that she pities Mami's emotional weakness. Mami is a legitimate hero figure who was cursed to be born in a world without heroes and that irony is clear to Homu.
That said, Mami actively tries to contract Madoka. While she'd rather convince Mami to side with her against Walmartnight, Homura can't do this due to Mami's insistence on contracting Madoka and this puts them at odds.
On Mami's end, she views Homura as a threat. Based on her experiences, Homura is a magical girl that wants to encroach on her territory. Why does she want to stop Kyubey from contracting the girls? Does she value grief seeds over people's safety? There is 0 reason to trust Homura and because most other girls (Kyouko for example) just come around and start shit, she can only conclude that Homura is scheming against her.
It's important to keep in mind that outside of E10, Mami and Homura have never actively tried to kill each other and instead go for restraining movement.