You say the flight is “operated by Delta”: that means it’s a Delta flight that happens to be sold by Aeromexico. Booking with the Aeromexico flight number impacts how you earn frequent flier miles, but otherwise you’ll on the same airplane either way. You can usually even see the real flight number if you search the flight details (AM 9989 operated by Delta as DL 167). You probably are best booking whichever is cheaper, or if they are the same price whichever gets you more frequent flier miles, or if you don’t care about that, book with the airline operating the flight.

For flights acrually operared by Aeromexico, they are actually a better airline than they get credit for. The onboard experience is roughly comparable to Delta, and all flight attendants speak excellent English. Most of the things Aeromexico gets dinged for, like the lousy transfer experience in MEX, wouldn’t apply to you as someone ending their trip in Mexico City.