PhD defense in germany: how do they decide for the final grade?
To me it seems that your defense will decide whether you will get the grade 1.3 or 1.7. You will certainly not fail. 

During my PhD it was roughly 2/3 the written thesis and 1/3 the defense, which is divided equally in your presentation and the rigorosum. However, for you I think this is not relevant since your Gutachten gave different grades and your performance will decide whether Gutachten 1 or 2 is "right". 

If I were you I would take the defense seriously and be prepared for the topic the professors are going to ask you. Often professors give some hint on the topic or might even tell you in advance. 

You should know that 1.7 will be rounded to 2, so I think it will be a "cum laude" vs. "magna cum laude". So it makes a difference. 

To my knowledge virtually all grades are summa cum laude (1+) (approx 20%-30%), magna cum laude (1) and cum laude (2). So cum laude is practically the lowest grade, although it is a 2.

BTW: For the summa cum laude (the distinction) you need 1.0 in both Gutachten and also make an excellent presentation, where professors test you for your limits. Professors often use it to indicate that they think you are ready for the academic career. Also during selection of some programs of the DFG it is expected to have such a grade.

I hope this does answer your question.