If you want Freesync, the 380, because the 280x doesn't support it (a little bit to old). The 4gb version of the 380, the 2gb isn't that much cheaper, and will limit you at higher resolutions and multi-monitors.
If you don't care about Freesync, and want pure performance, the 280x is what you want. It's more powerful, has a wider memory bus (and speed) and the 3gb of ram is enough.