Gerrit is ugly* and less fully featured, but I love it. It's great for linear master branches. I like the rebase support, it means what I reviewed goes into master, no squashing required. And diffing between rebased patchsets works great and is a killer review feature (most tools get very unhappy when you rebase).

It's only really for code review though. No nice web UI for browsing repositories, no issue tracking, etc. This was a plus for us (very unix-y and easy to integrate with existing systems), but could be seen as a negative.

* The UI is getting better, and you can tweak the CSS, it's pretty simple.

Wikimedia uses both, unfortunately. They each have their advantages, however, Phabricator has a usable interface while Gerrit is one of the worst GUIs I've ever seen.