Is there any real difference from an Amax other than a more heat resistant tip?
Yes. The ELD-X is the expanding version of the bullet. The bullet jacket is tapered, and gets thicker as you go down on the bullet. It helps control the expansion and prevent fragmentation and/or the bullet coming apart.
The ELD-M bullet jacket is one thickness throughout the entire bullet.
Also, the ELD-M bullets are held to twice the accuracy standards from the ELD-X bullets are. At SHOT show, Hornady said the ELD-M's are held to ~.12" and the ELD-X was .2" or .25" for the production runs to go forward. The ELD-X was twice the size difference, but he said they easily come in under their max.