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.