Should I migrate over to GitHub (from Gitlab)? Gitlab works just fine for me, and I prefer its interface over GitHub. But GitHub has a larger, more active community. There are very little "beginner friendly" issues available for me to work on in Gitlab land, which is pushing me to feel as if I'm stuck in a "tutorial rut". In terms of collaboration, when/if I were to reach out to someone in hopes of working on a project together, I imagine myself losing out on some great mentors because they don't want to create a Gitlab account.
The reason I moved from GitHub to Gitlab in the first place is truthfully, a) political reasons and b) CI/CD. Now with GitHub actions and a little bit of growth in the maturity department, should I migrate back over or am I just overthinking this?
Looking to build a portfolio that's public is fine on either GL or GH. If a particular mentor will not work with you because you're on one platform vs another, then switch. But GL is a perfectly good platform to host your public portfolio if you like it.
Digging a bit deeper, this likely falls into the rabbit hole of optimizing something that doesn't matter.
Focus on creating things that teach you - move you forward. Either technically or solve a specific problem.
What could change the above answer is if you're looking to build OSS. Less about building a portfolio but more about pure number of eyes. For better or worse, GH is still more widely used so then I could recommend a switch.
But I agree with ktpsns, git is decentralized. Feel free to use both. You open yourself up to more interaction and more safety for potential issues like service outages or data loss.
If you want to show that people "liked" and "followed" your repositories, you probably should go to GitHub, just because it has the biggest community.
Btw, there is no reason at all to host your git repos at a single site only. I mirror all my public repos between Bitbucket and GitHub.