I wouldn't swap my Sprenger System 4's for anything - I can ride and hunt and jump pretty much pain free now. Before I got them, jumping lessons were almost out of the question.
Think I'm getting the names right, but not 100% sure! I have the sprenger ones myself (black with blue bits on?), and do like them although they don't make a huge difference. Last weekend I rode a horse with the RR ones (with air cushion - ad has Anky and JW on it?) and they were superb, if I hadn't already got the expensive sprenger ones I would definately go out and buuy a pair.
Yeah i know someone with the Royal rider ones which she loves, but i dont know anyone with the Sprengers, they are the same price so im not sure which ones to go for.
I've got the flex royal riders, they have a good grip, but the flex bit is quite stiff relatively. So I find it easier to get my heel down with my metalab spring ones, but like the royals on my jumping wow as they are light stirrups.
I would think the system 4 sprengers will probably help more with foot position, but the royal rider flex will be better for grip and are much lighter.