I've got both 5520's and 5525x's, and the biggest differences between 5525x and 5520 are under the hood: multiple CPU's, more memory, more ethernet interfaces, higher backplane throughput, software IPS capability, etc. From the point of view of 9.1 firmware they should look pretty similar, except for the SMP image and increased interface count. I have successfully cloned configurations from 5520's to 5525x's and back between my test lab and my production networks.
Without seeing the two configurations it's going to be hard to identify what's different, and I completely endorse everything Marius said.
Note that without ACL's, the ICMP behavior would be controlled by the security-levels on the one hand and whether or not the global policy was inspecting icmp. Out of the box, pc1 can egress an echo-request because the security level 100 -> 0 transition permits it, ICMP inspection allows a matching echo-reply to come back in. pc2 is out of luck. Once you start applying ACL's, the security levels are irrelevant, though the inspection still matters.
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We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...