http://www.tankspot.com/forums/f97/51444-blood-draining-vs-blade-warding.html
An excellent thread. There are other threads on the TS forums but I feel that this one sums them up very well.
Here's a summary: Blade Warding provides almost no additional parries due to an obscenely low proc rate and up time. Blood Draining, on the other hand, procs many times providing us with much greater survivability. Will this change in the future? Only if our Effective Health increases in relation to the size of boss hits. Blizzard doesn't seem to like this plan, as Uld25 regularly cranks out 20-25k hits on tanks. The additional health gained from Blood Draining, at this moment in time, is FAR greater than the damage avoided with Blade Warding.
So in summary: Blood Draining > Blade Warding, until something big changes.
-Trismus
Thanks Tris, good information :D