comparisons - Hesitating between "as" and "than" to oppose two words

I am writing a letter and I am not sure about this sentence:

Being a parent is as much of a choice than a responsibility.

On various translation tools, it is suggested to use "as" instead of "than" but I find its use strange in such context. Does someone know which word to use and why?

Answer

Definitely "as".

These are the standard comparison structures in English.

... less ... than ...
... as much ... as ... 
... more ... than ...

I'm not sure what you find strange about "as" in this context but it sounds perfectly correct to me.