The ork growth cycle is fast, but not as fast as the Tyranid. It really will come down to the numbers and tactics of each race, the winner having a huge jump in population. I was reading the tyranid book, and they mentioned a couple things such as if two tyranids fight, the outcome is a larger force with the strengths of both at almsot no loss of resources (bio-matter). If orks are overwhelmed, their genetic strengths and genetic knowledge would insanely boost tyranids if the nids were victorious.