Obviously both method work. There are at least 5 skills: balance, pedaling, braking, steering abd situational awareness. I think the strider have the potential to be better with balacing and steering. I think situational awareness is easier on training wheels but it should be about the same.

I think that the problem with training wheels is when people leave them on for too long. I have also witnessed where kids misuse the balance bike they put their feet down or are constantly pushing off rather than ever coasting and balancing using that method isn't teaching much.

If you are doing the run with the kid I recommend the balance buddy. It puts you behind the kid rather than next to them. You can also vary your input without the kid knowing