I have a combined Synergy and MTS setup. RM100 with several Jaded Faith mods including dual channel module, foglifter, NFB, input gain, loop, and MM transformers/choke. I also use a couple of SYN2 units and a RM4 with my PS100 as a power amp.
The mods to the RM100 make all the difference. It really is what that amp should have been to begin with. Unless you find one already fully modded like I did or can do it yourself, it's better to go with Synergy as the base. Which base unit is up to you. Each has its own advantages and disadvantages; mostly with specific features.
For instance the Syn30 has a dedicated clean channel; which is a plus, but only one module slot and 30 watts.
The Syn 50 gives you a full 50 watts and 2 module slots, but no dedicated clean channel. It sort of defeats the purpose of 2 slots when you're almost required to fill a slot with a clean module if you need a basic clean sound.
Syn1 & Syn2 have all the features you would need and the Syn2 can be expanded with multiple units, but there's the extra cost of having to buy a separate power amp. I would suggest going with the PS2, PS100, or something else equally as neutral sounding. My preference is to let the module do the heavy lifting on tone and the power amp provide volume.
Of course there's always the option of getting a Syn30 or 50 and expanding the module slots with a Syn1 or 2.
As far as modules go, all of them sound good. So it's more a matter or preference of what you want/need. You also need to keep in mind that the modules aren't going to be a 1:1 copy of the real amp. They sound a good 95-98% the same, but won't ever be 100% exact. My personal favorites are the IICP and Pitbull. The IICP is excellent. I have a Mark III also. Side by side it's hard to tell which is which if you're not looking. Tone-wise they were really close. The biggest difference is the Mark III was more aggressive where the IICP was a bit more smooth.
The other part of modules is all of the older MTS ones will work with Synergy. Rob at Jaded Faith still mods them and will do one-off modules. I think Salvation still does mods, but they're kind of hard to get ahold of. Having the old MTS stuff modded is the best way to get a bunch of tones not offered by Synergy.
But say far, far, far away from the 1st gen MTS modules with Synergy. These are the one with the full chrome faceplate; no black stripe on them. There's a known, but no really advertised compatibility issue. I've talked to Rob/Jaded Faith and Bruce Egnater about this They don't seat well in the slot, will arc across the pins, and fry your stuff. I had one kill both of my Syn2's where I had to send them to Synergy for repair. Outside of that, all other MTS modules are good.
I have a couple of small gripes about Synergy:
There's no Syn100. They really need to come out with a 100 watt 3 module slot amp. If they had that I wouldn't have bothered with the RM100 in the first place.
It takes them a while to release new modules. I get that there's a good chunk of R&D involved and recent events slowed things down,, but its hard to hold on to the excitement for several years waiting for something new. Though I will say the IICP and 6505 were worth the wait.
There's no original modules. It's great they can replicate the tones of famous amps, but at the same time do we really need a dozen JCM from them. I really wish they would take some time and create a couple of fully original modules.
That's just small gripes though and doesn't take away from what they have going already. Overall it's a great system and better than the (unmodded) MTS. There's the initial cost of buying a main unit to get going, but after that you can build up some great, iconic tones at comparatively small costs. If you liked the MTS stuff overall I think you'll like Synergy. Their amps are much better quality that stock Randall ones.