Well I mean if you want to get technical;
On the surface, the covenant would wipe the floor with the Locust. Air superiority would really be the difference, since the Covenant would just use dropships and Banshees to destroy the Locust. The only air support the Locust have are the Reavers, which would give them somewhat of a 1 on 1 advantage because they're entirely too fast and agile for ANY Covie air weapon to actually hit them. They'd fly circles around Banshees and those plasma based weapons are entirely too slow, while the explosives that get used by the Reavers and the Theron Riders would absolutely destroy Banshees in a 1 on 1. The Locust don't have nearly enough Reavers to fly out and control the battle with as many enemies as the Covenant can throw into a dropship, and the pure amount of plasma/shots the Covenant could throw at a group of Reavers would be overwhelming. Not to mention the fact that, even if the Locust gained air superiority, they don't have anything big enough to make a huge difference in battles. The Reavers could give an advantage, but nothing like what a dropship could do. The closest thing to a dropship the Locust have is a gas barge, which is damn near useless outside of basic transportation.
However, there weren't really any specifics on how the fight's being played out. I mean, are the Locust being invaded in the hollow? Are they all being put on some random battlefield? Are the Locust being transported to Sanghelios or something? The Locust have a huge advantage in the way of being able to mine out tunnels and attack with surprise/guerrilla tactics. Taking that out of the equation isn't exactly a "fair" battle, since it's one of the major traits the Locust have. And, in an invasion-on-the-hollow scenario, it effectively wipes out the air superiority that the Covenant have, since they won't be able to get Dropships into the hollow. The scenario plays too big a part in how this would play out.
In a pure land battle, with no air support, I think it's pretty obvious that the Locust would completely decimate the Covenant. The lone advantage the Covenant would have is the use of Scarabs, but just how many do the Covenant really have? Not to mention, their weakness is clearly explosives, which the Locust have a ridiculous amount of between Diggers, Boomshots, and Torque Bows. A group of Therons could immobilize a Scarab without much effort. A lot of people say the Elite's would mop up the Locust because of energy swords, but you don't seriously believe an energy sword is going to pierce the skin of a Berserker, do you? And what good is it going to do against a Corpser? And on that note, is the Plasma of the plasma/energy weapons enough to burn a Berserker's skin? Because, if not, the Covenant don't actually have any feasible answer to Berserkers. Bullets and explosives are totally useless against them. The only damage you can do to one is by lighting it on fire first. Maybe a ton of Plasma-focused firepower onto a berserker could light up its skin?
Also, are we including the Lambent Locust in this discussion? They're enemies of the Locust and they refuse to work together, but are they part of this discussion at all? Because if the Locust can harness the power of a Lambent Brumak or Lambent Berserker, the Covenant don't stand a chance, Air Superiority or not. Obviously, a full on deployment of Lambent Locust is out of the question, but the Locust would be more than willing to drop a group of Berserkers into a battlefield by themselves, then do a gas-barge Immulsion dump onto them, forcing Lambent Berserkers. Same thing with Brumaks. They might lose 5-10 workers and a gas barge or two, but what kind of answer does the Covenant have to that? A single Lambent Brumak would decimate any Dropships in the area, whether by immulsion spray or by just eventually blowing up. It's like the ultimate trump card if the Locust are capable of making it work. The worst they would be able to do in any given battle would be stalemate and destroy all the land in the area.
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tl;dr - Where the battle is being taken place and what's considered "off limits" makes a huge difference in "who would win". Very different battle styles. And the Locust have a lot of things that may or may not work so well in a cross-universe battle.
Also, the Locust are way cooler. They can fucking transform when they get exposed to acid.