I touched on this several months ago here:
http://www.bluegartr.com/forum/ffxi-...n-tanking.html
Basically, the best test I can find tells me that the BASE damage reduction on a shield block is as follows:
Buckler: 22%
Round: 40%
Kite: 50%
Tower: ??
Relic: ??
On top of that, approximately +0.5% of damage reduction is added for every 1 DEF on the shield. Any additional VIT the shield might offer does NOT get included in this figure. Using this information, the following numbers can be approximated for the percent damage reduction on shield blocks for popular shield choices:
Simba Buckler: 22% + 12% (24 DEF) =
34%
Koenig Shield: 50% + 11% (22 DEF) =
61%
Terror Shield: 50% + 10% (20 DEF) =
60% (70% in Campaign)
Sipar: 22% + 10% (20 DEF) =
32%
Palmerin's Shield: 50% + 10% (20 DEF) + 2% (Bonus on Shield) =
62%
Acheron Shield: 50% + 12% (24 DEF) =
62%
Genbu's Shield: 22% + 12% (24 DEF) + 10% (Bonus on Shield) =
46%
When looking at this, one must take into account the size of the shield since it also plays a factor in the frequency of blocking, and any other beneficial stats on the shield. That being said, if you can get close to capping your block percentage, Koenig and Terror are the way to go. Simba and Sipar are really only nice for helping to cap your block rate on something harder (for recasting Utusemi on HNM or something).
For mage jobs, I'd say Genbu's Shield knocks every alternative out of the park as far as I am concerned because the block percentage will not be high enough to take advantage of its better reduction, even with the +10 shield skill. Genbu's shield has the additional benefit of giving -10% damage taken even when you don't block either, which is a nice plus for jobs that have poor, or no shield skill. An earth staff might still be better, but shit is situational, and all that.