Everyone in this thread who is saying that 100HP or 1% dodge is always best is absolutely 100% wrong. 100 HP is not best in every situation and 1% dodge is not best in every situation.
Since the OP was specifically asking about BWL, there are two things to consider. Broodlord and Chromaggus.
BroodlordHas multiple hard hitting nukes, on most private servers it is possible to be hit with 2 or 3 at the SAME TIME. He also uses mortal strike which reduces healing by a shit load.
There is also a very long and very annoying gauntlet before broodlord, you have to re-clear it EVERY TIME you wipe. The last thing you want to do, is wipe after engaging broodlord.
ChromaggusHas some very nasty hard hitting breath attacks. You can't dodge breath attacks or mitigate them with armor.
In the case of broodlord, the fight is SUPER EASY for 99.9% of the duration. For the other 0.1% of the time, he hits you with an auto attack followed immediately by a mortal strike, then before your shield block cooldown is up, he hits you with an auto attack that either crushes or crits. It's ok, you'll have another chance to see if you dodge one of those hits in 25-30 minutes when the raid is done running back, rebuffing and re-clearing the gauntlet.
You still want high avoidance on broodlord because dodge/parry on a mortal strike is awesome! The problem is, you have to assume that your avoidance is going to fail you at some point during that fight. I've tanked broodlord MANY times, in just-entered-BWL gear and full T2 BiS BWL. Even with the best gear attainable at that content level it was still best to max out your HP and armor on broodlord.
As for Chromaggus, the fight is easy until 20% when he enrages, at that point one of two things will kill you. A breath attack or frenzy. You can't do shit about frenzy, that ones on your hunters. The breath attacks can't be avoided, you need high HP to survive them.
In conclusion, 1% dodge is better than 100HP for 99.9% of BWL; HOWEVER, max HP is a REQUIREMENT for the last 20% of chromaggus and those moments on broodlord where your avoidance fails you. It sucks but you really need that extra 200HP. I also strongly recommend a
http://db.vanillagaming.org/?item=8411 for broodlord along with all the other HP consumes you would normally use.
Edit: If you want to be really pro, get a second set of T2 legs and helm, put 100HP on one, 1% dodge on the other. The dodge is better when you are tanking melee trash, the HP is better for most bosses because everything in BWL has breath attacks that can't be avoided.
