ShareThis

I've moved to ShareThis, for the following reasons:

1. I've always liked the branding and icon better. I think "Share This" has a much more intuitive appeal than "Add This", and I think the icon is an excellent representation of what's happening. (You may have even noticed that when I had the AddThis button, I called it "Share This".)
2. It lets me customize what services show up, which I really like. (Update: it turns out both services let you do this. See the comments for details.) I believe I've put the most useful and relevant networks for this audience on the first page - please let me know if something is missing!
3. I'm impressed with their marketing team.
4. This weekend I ran several speed tests, and it looks to me like they've resolved all the problems I talked about before. I think it could be as fast / faster than AddThis if I could get the code on a button level, rather than on a page level, but the timing difference is negligible now (and the configuration so much easier).