Always take the +27 Agility title for the base stat modifier. You gain more from it than you do from simply having 300 extra phys mastery, since the agility title adds nearly as much (maybe even as much, I'm not sure) to your phys mastery along with giving you some extra crit rating as well.
As a general rule, remember that base stats, like agility and vitality (when applied correctly), are typically a better investment than +phys mastery or +max morale, because they scale with level where boosts of specific types of modifiers do not.
In specifics, I use the Agility of Eldar Days on both my bow and main hand melee LI.
+27 agility, i guess because we are a DPS class and this way we gain the most DPS. It adds 270 pm and 54 critical rating. You could use the +300 pm scrolls too, but the +27 is just slightly better.