Security vulnerabilities involved with Base64 encoded url

I have an API, when trying to access it through a web application , I am making ajax get and post requests.

As I am new to this, I am wondering what the security concerns are with respect to the content type.

Right now I know of two content types I can POST the data as:

• Base64enoded URL.

• JSON

If I am passing the data as encoded url, the data will appear in the query string.

Even if Base64encoded would'nt it be a bad way of passing sensitive information?

I read several articles and most of them said JSON or URL encoded wouldn't matter much in terms of security.

What are the security breaches that I must be concerned with , when looking at the content type used to pass data between the application and the API?