These tools seem very similar to me in terms of text/file manipulation.
Can someone explain to me why I would choose one over the other?
TIA,
Jennifer
I don't understand your question. sed is an editor while awk is a programming language. Yes, if you want to change all occurences of "dog" to "cat" in a file, either tool will do. But you also could use c++ or vi if you wanted. It would be hard to find a programming language or an editor that could not handle that "dog" to "cat" task. But still, programming languages and editors seem rather different to me.
some people may argue that sed is a programming language as its turing complete.
Jen,
it is all what flavor of ice cream you like to have it now 
-ilan
The below seems reasonable...
##sed: a non-interactive text file editor
##awk: a field-oriented pattern processing language with a C-style syntax