M.S.
it really depends on your level of tolerance. i teach and can positively say that i am not, and i repeat am not suited to teach lower than around 7th or 8th grade (i'm a 9-12 social science teacher).
i'm not sure how the credentialing process is in tx, but in ca it is totally different as for primary teaching you'd need a multi-subject credential. for secondary you need a single-subject credential. but subbing first is smart to figure out if you want to get either credential.
to sub for middle/high school you need to be MUCH more savvy...kinda hard to put into words. but they will definitely try you in much more ways than primary in my opinion. my students come to understand that i am here to help them, but i'm not to be played with. there is time to laugh and joke, but when its not that time everyone needs to be on task. i generally give my students options....hard way or easy way (my way). good luck! there is MUCH more than meets the eye in this career.