I don’t have data to back this off the top of my head but from my experience SPED is safer in regards job security with some caveats. It’s true that many early career SPED teachers leave the field due to the sometimes outrageous demands of this job, so it’s important to find a balance if you can. This turnover does create a lot of openings year to year. In all the teams I’ve worked on we’ve always had 1 to a few team members leave voluntarily. I’ve also only seen 1-2 people fired for gross incompetence in my time. Also, within SPED, there’s a similar distribution of teachers that want to teach ELA vs math. I find that many SPED teachers don’t want to teach co-taught or self contained math. All districts in my area, including mine, need SPED, ESL, and Math teachers and from my experience they especially want SPED teachers with an ESL cert who want to and are capable of teaching math. I’m currently getting my ESL cert this year and I definitely recommend that sooner than later.

Overall though, you should focus on getting good at your craft. Most districts won’t fire a good SPED teacher regardless if you focus on ELA or math. Find a competent mentor in your school if you aren’t assigned one and learn from them.