I've had to compare the two. At the current state, Stash is better than GitLab in almost all ways. Better UI, more features and other small things that make a big difference (e.g. repo sync).

GitLab, on the other hand, has two advantages over Stash. Firstly it's open source (+free). Secondly and most importantly, GitLab is under active development and there is a new release with reasonable amount of content every month.

I haven't installed Stash, but for what it's worth GitLab installation and upgrades have been really straightforward.