Oh, and there's the stealth technology employed by both sides, so they could ambush the Reapers with mines if they're coming through the Mass Relays, taking out a large number before the battle could even start. Also, the UNSC and Covenant ships can take a huge amount of damage (Pillar of Autumn, although its upgraded), and the Reapers have issues taking down a ship about half the size of a standard UNSC Frigate. The Halo universe has slipspace, so they're able to escape that way and fight another battle, while the Reapers have to rely on Mass Relays to get anywhere, and the UNSC has ODPs, mass drivers, and MACs which would take out a lot of Reapers, while the Covenant have plasma, which I believe would destroy a lot of Reapers since they use kinetic barriers to stop projectiles, so theoretically, the plasma would go straight through, as well as the energy projector if its used in battle.
They ran the statistics through the sim, and the UNSC and Covenant (if working together) would win, but if the battle took place during the war, they would lose. Talking to a guy who I know that's going to Harvard for his masters in Computer Technology (not sure the actual name, just know it deals with computers), and he and a couple of his buddies made a simulator. I just finished playing the Mass Effect series last week, and I was actually wondering who would win.