I always liked full... Speed? Power? You know what I mean haha. If I had to touch up later or found I needed to hit my sensitive neck again to really put that finishing touch on a shave I would take it all the way down to it's lowest setting. It could be placebo but I did find I could send a little extra time on my sensitive areas without discomfort, not saying that I really experience discomfort anyway but every body has an area that they can push a little too hard if not being careful. All of the settings in-between highest and lowest seem pointless to me, I wish instead of a +/- it was just an on/off button for the 'sensitive' mode.
Not sure what to do with my series 3. I don't need 2 shavers and I don't travel enough to necessitate a back up nor do I have any kids or family members to pass it off to. My wife recently made a comment of, "I wish I could just shave my legs dry like you and not waste all this shower water" so maybe I'll give it to her to try.
I can’t tell any difference in the sound of the shaver or the strength of the vibration in each of the three settings. It could be that the difference you noticed shaving your neck area could be a placebo effect. It’s a very powerful part of our minds.
I don’t know if she will like using a men’s shaver because leg hair is different. Let her try it out and let us know how she likes using the shaver.