Depends. Would this hypothetically be used for a live or recording situation?
Live: Fractal all day
Recording: Synergy
On the Fractal side, I've only owned an Axe II, which still sounded great to me. Not sure how different the III is tonally as I don't really care since I've got off that boat long ago. The convenience of using a modeler live is just so nice and you can set your patches for each song, blah blah blah. Stuff you already know. The tone is great. Kemper is cool too if you have rare amps you want to capture and take with you, but I feel like the Axe does a better job modeling. I'll even throw a curve ball and say the Helix can keep up or even rival the Axe, especially V2. Again, can't compare to V3.
Tone wise, the Synergy stuff is spot on. This Synergy stuff is very under the radar and doesn't get enough love. All the modules I've owned were pretty spot on to what they were emulating. My only gripe is the IICP and UL modules graphic eq's were too sensitive and difficult to dial in. A very small learning curve but once you get it, you're set. Its not a bad choice for a live setup either but the lack of FX is what would deter me from live use. A simpler all in one unit is the better solution IMO unless you're willing to haul a pedal board full of stuff.