Fire Destro vs Shadow Destro

I'm not even going to pull numbers, or show proofs for this one. There are plenty of sites that show shadow usually beats out fire, and a few that show fire can keep up if you have mages.
The critical difference, is raid utility:
Shadow destro = ISB (improved Shadow bolt), a debuff causing 20% additional shadow damage to be done. Fire provides no such DPS boost to the raid. Shadow destro will cause affliction warlocks and shadowpriests' dots to do 20% more. Even if your fire destro lock can keep up in damage, there should be no question which is more raid viable. What is good for the raid, is good for the raider. Downing bosses > winning damage meters.