Unfortunately, I've been unable to use Unreal for game jams. The reason? Collaboration! I tried to get my team set up with Unreal Engine and git, but it ended up being a disaster where the team didn't have the time to figure it out. I also don't want to host my own subversion or perforce, and I also don't want to pay an arm and a leg for a hosted version.
On Unity, I pay $30/mo. You scroll to the "collaboration" tab, hit "sync" and your changes are in the cloud. It works great, my artist were immediately able to start syncing their changes.
I think in general Unreal is a more "professional, big teams" tool, and does not target the smaller market.