I am one of the people who has been through both, about 6 years apart. Your question is a tough one, because pain level is largely something that can be controlled by medication, so it is possible to go through either procedure with very little pain if enough medication is used. I would say that my pain levels were similar; maybe at worst a 2 or 3 for arthroscopy and a 3 or 4 at worst for resurfacing, but I did use a lot more painkillers for the resurfacing. Since the pain can be controlled in either case, a better question might be how disabling the procedure is for the first couple of weeks. On that scale, I would say that the arthroscopy was a 2 or 3 (though I was on crutches for a week or so), and the resurfacing was a much more serious 7, meaning that I was pretty wiped out for a couple of weeks. Also, I found recovery after arthroscopy to be a month ( which is when I started skiing again), to a few months (which is when it stopped hurting from skiing). Resurfacing is 6 months to a year before impact sports like skiing, and a year or two to be completely healed.
So my view is that the pain levels can be controlled so that they are similar, but resurfacing is a much more serious and traumatic surgery. In my case, resurfacing was also vastly more successful at getting me back to high levels of virtually pain free activity.