That's the trouble with asking for this kind of advice
. I look at it the other way than Zahgurim - I prefer avoidance (being an avoidance tank) over mastery. But in the end, both are viable. The key things are this:
- Mastery has no effect on CTC
- Avoidance directly increases your CTC
- Mastery has no effect on the size of your heal
- Avoidance has no effect on the size of your heal
Mastery shines on specific fights in Firelands, such as Shannox, Beth'tilac, and Domo. Avoidance really comes in handy during Decimation Blade phases of Baleroc since you cannot mitigate that effect and Alysrazor hatchlings during tantrums. Some DK's will carry around mostly mastery gear with a couple of good avoidance trinkets and bring those out for specific fights.
Since you do seem to be going for mastery, I might think switching out the Swordshattering for Stoneskin would be a good choice. The reason behind this is that you're sitting at 35% avoidance, which is really going to start fighting your mastery; I'm at 43% myself AS an avoidance tank.