I want to say the Romulans would win. However I think the Romulans are more in to posturing and saber rattling instead of instigating an actual conflict. Sort of like North Korea today. They have the technology, and manpower but no one is serious enough to pull the trigger on a war. The closest attempt was when Shinzon was praetor. Donatra and other Romulans supported Shinzon's coup but got cold feet when they realized what the victory over the Federation would look like. Earth's complete annihilation by use of a biogenic weapon.
It's like they intended to rule Earth and the Federation worlds not kill them all. Makes you wonder what they were thinking when they first supported the coup. You would think casualties in the millions would be implied by the very notion of interstellar war.