The 365s have no air conditioning at all (cabs excepted). It's just air being forced through (and that doesn't always work).
When the forced induction does work it's arguably better than having the windows open, as you can direct air on you. But that then requires the ball vents to work.
Nice as the 365s are, they're NOT good on hot days. Sure, at full pelt and when they've had a chance to bring air in (and you've also cooled down after rushing for the train) they can be fine - but the windows create horrible noise and definitely making it uncomfortable for people in certain locations.
For some reason enthusiasts love the noise almost as much as an old HST engine. I think most would agree that air conditioning and sealed windows are preferable. The air con on a 700 is also loads better than older Siemens stock - it being far quieter for one (almost silent).
The 700s have been pretty awful in this recent bout of warm weather. Having rushed to catch a train, I can assure that sitting on the right-hand side heading into London at midday on a sunny day is not a pleasant experience.
I'd take opening windows *any* time over air conditioning, simply on the basis that I'd prefer some flow of air compared to the potential prospect of non-functioning or ineffective air conditioning.
As far as etiquette goes, I'd say it's another one of those things where he who arrives first should get his wish. I don't understand why some people seem to arrive on a lightly loaded train, choose to sit somewhere where others have windows open or closed, then make a big thing of wanting everyone else to bend over backwards to accommodate their own preference. I've found over the years that a small ball of rolled up newspaper wedged in the right place makes any prospect of confrontation disappear, as strangely those who make such a fuss about things will then either stomach it (in which case it wasn't quite so much of an issue as made out to be), or find somewhere else to sit (which perhaps they should have done in the first place)! As people have said, the issue will go away when all trains have air conditioning - we just have to put up with mobile greenhouses instead.
None of this is unique to trains though, there's just as much bitching and contention in your average shared-use office when it comes to temperature. If anything worse as there's less scope for people to move elsewhere if two people want different things.