Mass Effect has an advantage in versatility, between various advanced weaponry, biotics and tech powers, as well as a larger army with more races (I think) against enemies that effectively amount to an army of soldier class. I still struggle to believe the Covenant's weapons are inferior to their ME counterparts, but suffice it to say that ME's guns are more advanced than (most) of the UNSC's. As the Covenant I'd make liberal use of Drone swarms. Their plasma and needle weapons would control the skies, an otherwise impossible notion if no spaceships also includes fighters and Banshees. But to answer the question I don't know who'd win. ME has versatility and advanced weapons, in which the Covies aren't complete pushovers, and the Covies have sky control by default. I don't get the notion that it'd be a curbstomp in ME's favor, but I could see it swinging their way.