Definitely standalone shows. I personally love to be as near to the stage as possible, and at a festival that'd be all the harder to do. I also like the fact that at standalone shows everyone is there for the same reason, I love meeting other fans at shows and that atmosphere. Especially if you queue from the morning, you're guaranteed to meet people who are about as obsessed and into Green Day as you are, and it's often the only opportunity to meet such people in person.
That said I find the festivals atmosphere just fun and chill (well, I've only been at small-ish festivals in France, so not sure how the massive ones would feel), and I'd totally love to see them at a festival in France... but I'm just not as willing to travel for a festival as I am for a standalone show.
As for the size of the venue, I find each size has its charm. Seeing them at Bataclan along with other ~1.5K lucky people was really special, but so it was seeing in the Giants stadium in NJ with ~80k people in the crowd. I'm really not picky, I'll take any format
Say that to the lucky 50 or so who got to see them at that tiny pub in London last year