The biggest difference is the community around them, Gogs is more or less the brainchild of Unknwon and he used to have a tendency to just drop of the planet and Gogs would sit still for weeks (or even months) at a time without anyone being able to do anything about it. Gitea was a community response to this, nearly all the major contributors switched over and I think it shows in terms of features and fixes. Gitea now has well over a thousand more commits than Gogs and a lot more manpower behind it. You can see this by looking at the contributor graph for both projects, Gitea has had a steady supply of contributors and commits in the last release while Gogs has a few spikes of activity.

Also, if you look at the release notes for both of them I think it's fairly easy to see that Gitea is developing at a much faster rate and has more features than Gogs already with a lot of nice features coming in 1.5.

However, I'm fairly biased, I used Gogs for a long time but switched as soon as Gitea popped up for the reasons above. Since then I've contributed a few fixes and features to Gitea as well.