It is almost always better to transfer MR points to an airline's frequent flier program. You might be able to buy the same flights using any of the below methods getting more value for your points.

In your case, Amex MRs won't transfer to United, but Chase points will. If you do have Chase points, they will transfer to AC Aeroplan with a 20% bonus through end-July, 2023. If you don't have Chase points, Amex MRs will transfer to AC Aeroplan.

Going outside Star Alliance, Amex MRs will transfer to DL for either DL or WS flights.