Andy I think if you asked this on different forums you'd get different answers.
The Hot Toys neck is great for creating dynamic, lifelike poses. You can create the most subtle impression of your figure being human just by altering the neck and head angles ever so slightly. I think this is better for people who pose their figures in 'action' poses, and constantly change them
However, as a personal preference I like to find what I consider to be (at the time) the perfect pose for that character, and leave them in it. And then the most important thing for me is that the figure look as close to 12" version
of the real people. And for that I need adams apples.... and necks with muscle tone sculpted on.... and
no cut-offs directly under the head. There's nothing more off putting to me than seeing a decent head sculpt where the face/head is great... and then where the neck should start there's a complete cut off line. It's hard to maintain the realism if the neck shows like this.
I guess it's similar to arm/let joints showing, the way people complain about that.
Of course this neck thing doesnt matter if your figures wear neck/head covering outfits (like modern military figures) and for that the HT system is probably better (like I said... go to different forums and you'll get different answers)
but for me... I prefer the Sideshow neck way. More realistic to the eye.