pero or sino que

One example drives me crazy (again). When to use pero or sino que. 

Example: Luis y yo no hemos terminado, ....... decidimos darnos un tiempo para pensar en lugar de tener que romper.

My answer = pero, because it is correct and I asked as well native speaker and they would use pero.

Strict rules instead of colloquialism @kwiziq? 

In 99% of all cases pero is correct but not in Kwiziq. 

Would it be possible to rate the answer as correct but add a hint stating that the strict rules would rate differently?

Thanks,

bernd