This is a copy paste from another thread, but I think the info applies here:
I have both the Metro Plex, the Synergy Plexi and Synergy 800. They really are all great and of course similar in many ways, but the differences are noticeable.
First off you really can't go wrong with any of them. If I were to keep only one module it would be the Synergy Plexi.
The Metro Plex to me is the most vintage feeling, a bit looser on the low end. I'd say even though it's based on a Plexi, it has a slight JTM 45 feel, but with much more gain available. The EQ works well, plenty of range.
The Synergy Plexi is tighter than the Metro Plex, and I really like the ability to dial in both Volumes, it gives you a lot of options tonally. Again this one has a very useful EQ, they've really dialed the EQs in on these modules.
The Synergy 800 has the grainiest overdrive, in a good way. I know that JMP/JCM 800s have a reputation of being a bright amp so initially I wasn't sure why the module has bright switches. It turns out I really like the differences between them on and off, and I use both. Surprisingly though the gain feel is a bit different, I think total gain wise they're all pretty similar, you can get them all to sound quite alike.
I've kicked around selling the Metro Plex module but for now I'll probably just keep it, as they all have their unique qualities. Hope that helps and let me know if I can get more specific on anything.