Side Stage is great
I was side stage SEC 15 for one of the spring Boston shows & also directly behind stage (1st row) for another. I was much happier being side stage.
IMHO - the best seats are in sections 15 and 20. Well, the best value per ticket price. I'd rather be paying 2 x 95 for SEC 15 or 20 than a zillion dollars for SEC 21. Even the scalping prices for the side sections are a much better deal.
Sound on the side is much better than behind. You have that side stack and it's just much easier to hear exactly what Bono is saying / singing. In the back, the sound is a bit weak and muffled.
In addition, on the side of the stage you get a great look at the light show, floor light rings and side projectors.
Lastly, I don't think behind the stage in SEC 17 or 18 is bad. In fact, I'd rather be there than across the arena and high-up. Heck, behind stage lowers are better than anything directly across IMO. That might be partly influenced by my desire to see the interaction between band members.
Well, good luck. Hope this helps!
Dave