Short answer: You should probably go for Mastery instead of avoidance, but if you are not really good with your Death Strikes, then avoidance will probably help you out more. Go for whichever fits your level of skill/playstyle. But do NOT try and even them out. Go for one or the other.
Long answer: There are lots of references in multiple threads to this same topic. Would link them but it would take too long to find them all.