If you didn't get a teaching position, which would be the next best option substitute teacher or teacher's assistant?

After subbing
my way through college - it is the one job I will never do again.  As a sub - you have no
consistency.  Depending on the size of the district you work in - or how many schools you go
between, the kids never really get to know you and you never have an opportunity to develop real
relationships with them - which is (in my opinion) a teacher's best chance of being respected. 
I only worked in two high schools, worked almost every day, and even saw many of the same
students over and over, but something about the fact that I was just "babysitting" the
class for someone else made it impossible to ever feel affirmed and relevant in the
job.

Even if being an assistant teacher pays less - it is consistent.  You
will be in the same room, working with the same kids and teacher - you will feel relevant and
useful, you will probably get better at what you are doing in ways that will affect your future
in teaching.  While subbing seems like it is perfect for networking - I
think teacher's assistant would be better.  It is better to have two great references from your
lead teacher and possibly the principal - than to have 100 "Oh, yes, I think she subbed for
me once" recommendations.  That's just me.  They would have to substantially increase the
sub-pay nationally to get me to do it again.