Well maybe it's been done before but I haven't been on Heresy long enough to see one, so I have no trouble with this.
Spartans are probably more agile, likely just as strong, and have the advantage of shields as well. But their ballistics weaponry probably has the same statlines as stub guns and their close combat armament is lacking.
Tough call but... marines.
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A World Eater survives another day at the office (barely).