its a question of quality vs quantity..
Clone/Storm troopers are mass produced with minimal skills and covered in essentially worthless armor that's only practical purpose seems to be to be used as a disguise against the empire... However there are a ton of them, so they rule the universe....
Spartans are trained from a young age, their armor costs a fortune, is the best the UNSC can make, and has never been stolen by their enemies for use as a disguise. Their training and conditioning is the best possible, and on a 1v1 level, they own stormtroopers so hard its not even remotely funny....
Having said that, who made the better investment... the UNSC on a handfull of spartans, or the Empire on legions of stormtroopers....
Very debatable, just about all spartans died in a battle over a single planet, however the surviving one can defuse a bomb, jump out of an airlock, throw the bomb into a gigantic alien ship, blow the crud out of it, land on a frigate, go to earth, wreak havoc, and then wreak more havoc.. If the empire lost a planet.. bleh, they lost a battalion, they still have a few million more.. It's not like losing the death star set them back much...
So whats better... putting a few baddass eggs in one basket... or mass producing basic soldiers..
My feeling... UNSC won against amazing odds.... the empire failed with the odds stacked in their favor, Spartans win. Stormtroopers are just cannon fodder so the heroes of star wars can slaughter and prove their skills by anihilating masses of inadequate (but very cool looking) troops.
(btw I love both and would love to have a set of clonetrooper armor, I just don't have the money)