As the owner of a V8, I voted Mesa. Much more sophisticated eq and sounds every bit as good. I loved mine even if it killed me to lift it. The eq on the Trace V8 lets it down a little if you team it up with a harsh sounding bass. Never liked Ampeg SVT's just couldn't get a clean and honest sound out of them. Never played an Orange so can't comment on those.
Tip on buying used Mesa 400+, as well as checking when the valves were last replaced, also check the condition of the transformers. If they show signs of significant rust then walk away. Mesa didn't coat their transformers and any corrosion weakens them by bridging the gaps between plates.