I'd go for the Cabal in general. Both groups are space empires who conquer and subjugate other races and are known to blow up/glass planets/solar systems for various reasons after first assaulting and destroying various ground and fleet based infrastructure with seemingly endless amounts of troops and resources. It's actually a bit difficult to compare who has more soldiers/ships/resources what have you as both are rather vague about that kind of logistical sense and the actual answer for those kind of questions is in reality however many there needs to be to tell the story. However there are some key differences between the two that puts favor towards the Red Legion. First and foremost, technology. The Covenant are specifically noted to imitative in their technology rather than innovative. Everything they create is based off a forerunner design and they do not make any attempts to understand or improve said technology as it goes against their religious teachings and ultimately their technology comes down to swords, shields, ships, and guns. If I recall correctly the UNSC is more advanced then the Covenant at the end of the war. In a direct comparison the Cabal are noted in game for being able to analyse and comprehend Vex tech, have sensors on every soldier to completely analyse the battlefield in real time, are able to analyse and reverse engineer Hive magic into a techno-logic base in order to capture the Traveller, [spoiler]Do the same to the Taken in order to steal light from the Traveller and empower Ghaul, and are implied to have built both larger then planet sized ships on their own.[/spoiler] That they make some of these improvements during the game implies an active use of science and engineering to improve their capacity that the Covenant simply do not have, and would be a major advantage in any conflict between the two. The other big advantage that the Cabal have is physiology. The main force of the covenant by pure numbers, if I recall correctly, are the Grunts. While the main force of the Cabal are...well, the Cabal. They are just stronger, tougher, and have less tendency to panic in general. I can't really imagine a scenario in which the Grunts could defeat Legionaries that did not involve the Grunts outnumbering the Cabal by at least an order of magnitude. While Elites and Brutes are probably of similar height to an average Legionary, the Legionaries seem, to me at least, to have more mass in general which would suggest more strength and endurance in general. Hunters certainly outclass legionaries in terms of power but that's rather easily matched by the various Colossi and Valus the Cabal have, who are all combat veterans in their own rights. The Cabal just in general more physically powerful. Then you have to take into consideration the Psions, who fill roles similar to the Jackals, Drones, and Engineers, are probably all around better at said roles do to the fact that they have psychic powers with the strongest of Psions capable of altering the orbits of heavenly bodies (Psion Flayers). I'm sure there's other things I'm forgetting but those two points certainly point to an overall Red Legion advantage.