Low potency/high rate:

+ Lots of Shield Swipes, good for TP management and occasional Pacification effects (though this is rarely truly useful on anything not trash)
+ Consistent dependable reduction
- Weak
- Benefit the least from Bulwark
- Most tank deaths are from spike damage and not attrition; a low potency shield is less likely to save you
- Some Shield Swipes will be wasted

High potency/low rate:

+ Most likely to save you from spike damage
+ Combines with Bulwark productively for effective -18~32% damage reduction on blockable attacks for the duration
- Not many Shield Swipes, leading to rare TP starvation issues when threat is established

Middle is obviously between these two extremes. I personally feel that you should be using mid potency/mid rate or high potency/low rate. I am not a big fan of low potency/high rate for the reasons above.