Originally Posted by The Glyphstone
Waaaughs don't grow spontaneously, though - their 'growth' is by attracting more warbands and smaller Waaaughs are reinforcements. Individual Orks might become bigger and stronger in response to the fight, but their numbers won't multiple appreciably without additional forces. The Ork life cycle is actually rather slow in regards to growing new orks from spores, and likely much slower than the time it takes to gestate a Zerg larvae into a new combat organism. If the two sides are calling in outside reinforcements in that manner, the Orks are going to get flattened under the weight of the entire Swarm, and if it's limited to on-planet attrition, the Zerg can replace their casualties faster than the Orks can.
Well it depends on whether you go by fluff, or in game.
By both in game mechanics, Orks and Zerg grow within seconds, the difference being that in the game mechanics Orks spawn in entire squads already ready to go.
If we go by fluff, both takes weeks to months to grow at any appreciable rate with orkish true regeneration coming in at the yearly period.
Zerg, from gestating their new brood, and Orks from drawing in more Orks from the WAAAAAAAAGH! and their spore replenishment.
In raw man of the line durability? I gotta give it to the Orks, a lack of definable anatomy, extreme endurance, and physical strength exceeding that of Terran Marines (Just going by the higher end lifting and resisting feats documented in fluff. An armored Terran Marine's lifting capacity caps out of the tonnage range, whereas a Space Marine can physically lift a Lemann Russ armored given proper leverage, and the equivalency of Orks to Space Marines in most physical attributes.)
However, they lack all the decidedly "killy bits" as default to compare to anything more than a Zergling naked.
I'd hazard that an Ork equipped with a shootah could easily compare to a Stim'd marine though, with the slightly lower rate of fire being compensated for in raw explosive force, and a big shootah going even further than that.