If I remember right, the rule I've used for solo healing heroic content with Shield Ally is that 1% uncontested avoidance is roughly equivalent to 2% heal crit. I emphasize 'uncontested' because, as you've seen if you've read the eq2flames thread linked above, that is the only avoidance that works with SA. Normal avoidance from defense skill or AGI doesn't help at all. Of course, that's just a rough approximation, and it'll vary a lot depending on the content you're taking on and your group setup.
In the end, that puts the AGI line pretty well in balance with the other Cleric lines for heroic content. I don't think you'd be significantly better or worse off if you take the AGI line instead of STR, STA, or INT.