Good question, and an issue that causes a lot of confusion on this course. There are two issues. One is that when you hear a sound that does not match exactly with a sound you are used to, your brain tries to match it up with one you do know. In this case (and I have listened to it carefully) I think it it between your two suggestions.
The second is that this pronunciation is not the one given in most books and courses, except as a regional variation. But as it is the form found in areas where Gaelic has survived the best, and because (according to Joanne) it is spreading geographically, you had better get used to it, while remembering that many people still pronounce r in the way you might expect.
Books tell you that it is pronounced /ð/ (like the th in the), but it quite often seems to be more like a /θ/ (like the th in thing). In addition, I think this speaker is pulling the tongue back a little, as if trying to say r. Try this and you will find that if you pull the tongue back too far it begins to sound like an s. So I think you can describe this sound here as a /θ/ with the tongue pulled back almost far enough for it to begin to sound like an /s/.
But assume it is a slender r if you hear anything between all these extremes, but don't feel obliged to copy them. D