An issue would be they come from very different eras, making direct comparison tricky.
Still... lesse, Tholians have Web, Breen have energy dampeners. Webs seem to be able to set up at a good distance. Energy dampers are a big advantage, *until* one adjusts to compensate.
My gut says Tholians. With the webs, they don't have to engage too directly and aren't defenseless even when drained, and their tech is kinda odd and advanced so I figure they'll be able to compensate relatively quickly.
I always felt that once the Feds got around the Breen energy weapon, they were much better able to handle them in a fight, and the Breen "threat" diminished some (though not completely).
Though obviously the Tholians won't have that advantage. Not saying the Breen are shit, or anything, just that, outside of their energy weapon, their ships are average at most.