MCU Spider-Man would absolutely side with Miles in Across the Spider-Verse
One of the core features of No Way Home was trying to save the villians that had been transported to the MCU. And one of the other main parts was Doctor Strange and Spider-Man disagreeing about whether or not to save the villians or let them die. Spider-Man was trying to save them because he believed that they would continue to live if they were saved and transported back to their universe.
This aligns with Miles's belief that he can save everybody. (You have the choice between saving one person, and an entire world-every world." "I can do both!") Both MCU Spidey and Miles believe that they need to save everybody no matter what.