Habitat Quality: Distance of threats in the habitat/cover

Dear InVEST team

My name is Diego and I have 2 questions related to distance of threats in the habitat:

1) According to the InVEST Guide, the distance of the threat in the habitat can be either Exponential or Linear.

Based on that, the questions related to the distance of the threat in a cover/ecosystem/habitat, etc. are formulated.

However, when consulting the supplementary material of 2 papers, I have found the following approaches.

I don’t know in which of the 2 there may be an error in the approach, but following the InVEST guide then the correct formulation would correspond to image 1 right?

2. Regarding the calculation of the distance of the threats in the cover or habitat (euclidean distance):

I am working in QGIS and I have found these options to do it (see images below) but I don’t know which one would be more accurate in this case.

Thank you very much for your help, this would clarify a lot of things in my research.

Best regards,