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).
But this is not the latest technology trend. Pls advise customer to check with the PCB vendor directly for latest product spec
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
Cost : different material may incur different cost. Do you need the best material but it will cost the most expensive ?
Design requirement - PCB trace length : If using lower grade material like (Megtron 6) still can meet overall transmission loss, do you still need to upgrade to (Megtron 7)
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.
