With the job market the way it is right now in teaching, I would opt for the Special Ed certification. I have 3 different certifications, have coached junior high, have life experience and still do not have a full time teaching job. I think it will be real tough to just get a coaching job with so many teachers and coaches hunting for jobs. Plus, you have to work the system and become known to the coaches and AD's.
Good luck with you search and make sure you have a lot of options available to you once you graduate. Work with your school's placement office to find a position.