Given blocking was nerfed Pre-WOTLK with the timer being heavily extended you'll get more use out of the 40BV as that will count on all your shield slams, than the piddly damage boost from a spike when you are blocking.
Given tweaks to healing, and supposedly Naxx also being tweaked according to a blue on MMO, if you are defcapped, have decent exp/hit I'd personally make sure you have the basic survivability for job in hand, then look at this more seriously later when you've farmed some 10/25 gear. Not much point blowing cash on gear that will probably get upgraded, if it ain't broke and all that.
Former TBC/LK Tank
Waiting for Dunmail Jnr to sleep to enable online gaming.