PCB material spec will gives you some spec like "what's is expected loss over the PCB trace length" (for example, refer to attachment pic for some spec comparison between different vendor).
Yet, actual PCB material selection maybe affected by other factor as well. So, there is no definite answer on which material to use. It's really up to your customer design requirement vs board development budget
Based on below article, Megtron 7 may has 20% performance boost over Megtron 6. Megtron 6 may bump into performance limit for 25Gbps over large format backplane.