Hmmmm.
Recently I've been converted to the "bristles that lay flat". I use the Gardiner supreme soft, and it does similar.
Me reasoning is that a cloth can agitate the dirt on a window very well, and that is basically like a flat brush head. No bristles on that etc.
Plus the splay allows you to get right into the corners to flick out spiders, and meanwhile the dual trim part of the brush has still got its bristles pretty upright in the glass.
I'm looking forward to the Gardiner ultimate brushes mind. From what I've seen they have less splay, so should be good for leaded as well.
Think I've got more brushes in my garage than a hairdressers.