Brett has asked for feedback until Dec 1st, so here are my quick random thoughts:
1) GitHub's UI is more polished than GitLab's.
2) GitHub pull requests are not universally liked (see famous Torvalds rant "... pull requests and the online commit editing, are just pure ...").
3) Attracting contributors? This was also an argument for the svn -> hg change, but it did not happen IMO. On the contrary, patches like https://bugs.python.org/issue3526 seem to be more rare.
4) Personally, I probably would not have contributed to Python if a GitHub account had been required back in 2012. But the time may come when one is excluded entirely from any Open Source contributions without a GitHub account...
5) The "do-nothing" option hasn't been mentioned: The current setup is actually quite nice compared to almost 99% of other projects. I like it better than both GitHub/GitLab.