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.
First of all, the majority of the North Korean military is outdated, its weaponry is all Soviet-era, inferior to the weaponry of its US backed neighbors. Also, its economy is weak, and would be incapable of supporting a war.
The Romulan Empire seems to be filled with internal strife, as a sustained conflict continued on, rival factions with varying opinions on a war with the Federation would take action against each other. Political instability would grind a war effort to a halt.
Also, the Federation seemed to be considered one of the three major powers in the Alpha Quadrant before the Dominion War, meaning it was militarily on par with the Klingon and Romulan Empires. It was on par, despite the fact that it maintained a fleet of antiquated vessels, more concerned with exploration than defense. After the Dominion War, the Federation's military is a lot more powerful, with newer, more war oriented vessels.
If the Federation was on par with the Romulan Empire when it was busy exploring in old ships, could it really be weaker than Romulus after radically improving its fleet?