So last night I my second piece of Arkanian Survivor's gear. While messing around with it I noticed that I can stack the 2 piece set bonus from the Arkanian on top of the 2 piece set bonus from the Rakata (lvl 50 raid gear). That causes your Kinetic Ward to give 25% bonus to Shield Chance instead of 20%. What I'm wondering is if the 5% extra shield chance is more or less useful than the 2% damage reduction from the 4 piece set bonus.

If the extra shield chance is more useful, when does it stop being more useful (taking into consideration higher armor values, extra endurance, etc...)?