I was at Boston II and Boston III. Personally, I enjoyed III more than II. Why? Electric Co, An Cat Dabh, Into The Heart & Vertigo twice. I'm hardcore U2 (like many of you) and I wanted to hear Electric Co, An Cat Dabh, and Into the Heart live. I also happen to be insanely in love with Vertigo live, and was thrilled to have it close the show in addition to it's #2 slot in the setlist.
I dare say that song selection to most fans seems to be the critical and defining element in determining which show was "better". In other words, a fan's song preference is huge when picking the "better" show. The common theme I see in the posts hear is the setlist night 2 vs. night 3, with night 2 preferrable to most fans vs. night 3. Night 2 gets the nod because folks posting here prefer Gloria/Ocean instead of An Cat Dabh/Into The Heart and they prefer an spontaneous OOC in addition to OOTS instead of Party Girl/Vertigo part deux. Really explains why I prefer III to II...it's because I preferred night III setlist to night II. That really seems it in a nutshell.
Performance wise, both nights had some screwups, night 3 perhaps a tad more than night 2. Personally, I enjoy the screwups, makes the performance more "real" to me and less robotic. It's been my U2 concert experience that when the band is "perfect" in their performance, I'm less inclined to have enjoyed it as much as those concert performances that have flaws in them. Call me crazy but thats how I look at it.
I also give III the nod because I was surrounded by less casual and fair weather fans during Boston III and more diehards. Let me tell you, it DOES make a difference!
So, I give the nod to Boston III over II with both shows ranking high on my all-time great U2 shows.
