Fwiw, I have both a Diablo and a couple of Joysticks and I tend to favour the latter as a helmet light partly because the bean pattern is more of a spot, which I prefer for a helmet light – I guess the depends a little on the model of each light – and partly because I find it more comfortable, particularly on bumpy stuff where your head bobbles about more.
Weighing them, my Joystick is 86g and the Diablo is 113g, which doesn’t sound much. But I think that the way the otherwise excellent Exposure helmet mount sits the light an inch or so above the top of the helmet, means there’s a sort of lever / pendulum effect that exaggerates small weight differentials. By contrast, I barely notice the supplementary Piggyback cell that sits low at the back of the helmet.
Not saying that the Diablo’s not an excellent light btw and I like on on really nadgery stuff, but mostly I seem to end up carrying it as a supplementary back-up that works either on the bars or the helmet and 99% of the time for me at least, the Joystick plus a big floody Lumicycle bar light, is all that I need. YMMV obviously.
Edit: also, I find that running a really bright helmet light, screws with my depth perception – hard to tell the difference between a shadow and a two-foot step etc, but maybe that’s just me.