Giant Barrier

I just got Giant Barrier and Im wondering which enchantment will be better, time or penalty. Is it better for it to last longer? What does penalty mean? Does that mean that for each + it will proc at a higher % of hp? So at +10 it would proc at 30% hp instead of 20%?

true, at +10 (decrease penalty) you can use it at 30% HP

but go for + time (20s of immortality at +10 :-))