Neither are better, it can only be better for a certain style notoverall they are balanced trust me.
Only you can manipulate the two different factions and leaders to decide who is better for you but besides that what the other guy said.
Covenant are a bit better for rishing as they have leaders etc, but they can be easily countered by the UNSC IF the UNSC character is a good enough player. The covenant also have good cheap units to buy at the beggining of a game. Brutes, Elite Honour Gaurds and Suicide Grunts, all are effective if used correctly and quickly.
Although, the UNSC are better in longer games also said before on this thread. Purely because they have two special units, one that you get at the start like Cyclops, Gremlins and the Elepahant. But you also have things like Grizzly's, Hawks and ODST's, which obviously take a lot of upgrading but would be better in the long run.
So no one can really pin point who is better for anyone but themselves, its all about your own style of play and the way you prefer to play.
RDC II Outlaw
Gamertag - RDC II Outlaw
Currently playing Halo Wars, Cod4 and Gears of War 2