Keep them both. A lot of people assume that one set of tanking gear is enough, but that's not always true. Regardless of which side you choose in the Stam vs. Avoidance debate, you have to realize that for certain fights, having a bigger health pool is better, and for others, having more avoidance is better.
With that said, make sure you have pieces for both. I use Outfitter, and I've set up 3 or 4 different tanking sets, depending on what the situation calls for (stam vs. avoidance vs. high threat, etc.). Sure it might be overkill, but is sure makes tanking certain encounters much easier.