After reading through this thread, one question comes to my mind: would it be safer to wear a bmx helmet than one of those traditional style helmets regardless of the style of unicycling (i.e. street vs. casual riding)?
Off the top of my head, I think the answer to that would be yes, not just for unicycling but bicycling as well. And a football helmet would offer even better protection, and a full-face motorcycle helmet would be eve better. 
That said, it boils down to how much helmet are you going to wear to match your activity? A road or mountain bike helmet is made to be a good compromise between comfort, weight and protection, meeting the industry standards for what types of impact it must be able to take. A skate or BMX helmet is more protective, but is also heavier and hotter. It could be very inappropriate for long or hot rides.
I should have a skate helmet for some of the riding I do…
BTW I have hit my head on the ground from unicycling, quite hard, more than once. The most recent was in the 15 mph Coker-shoelace crash that broke my collar bone. Shoulder, knee and head were the main points of contact. My helmet (the one in my avatar as I write this) served me well.
I’ve been using a normal cycling helmet when doing some Muni and longer road rides, but was considering getting a skater/bmx helmet… I suppose for the “cool” factor in that it just looks better. But you make a good point about being them heavier and hotter. Rekon I should rather spend my money on a better cycling helmet, my current one was rather cheap and I don’t trust it that much but it’s better than nothing.
I have a conventional bike helmet, and a skater style bowl helmet. I wear the former for long distance road, commuting and light XC where I don’t ever intend to hit my head on the floor. I wear the skater for trials and serious Muni where I can pretty much expect it’s going to happen at some point (and it has). I think this is a good compromise between protection and weight/heat, and also saves my relatively expensive, one-hit-only helmet from needing replacing often as the relative cheap multi-impact helmet has taken all the hits so far.
Before you rekon too much, consider that pretty much every bike helmet on the market has to pass the same safety tests to prove their “strength” before being certified by the various organizations that keep track of those things. A $20 helmet will withstand as much impact as a $150 one. What’s the difference? Basically you’re paying for up-to-date styling, and more holes.
That’s bike helmets. When you move on to skate or other helmets, the strength standards are different. I’m assuming a skate helmet can take much more abuse than a regular bike helmet. Plus skate helmets can be used after repeated impacts, where lightweight bike helmets are not intended to be used after a hard crash.
I wear a skater helmet ALWAYS…
After a fall i had last night (I am unsure if i hit my head or not) i am glad i had it on.
My helmet will never leave me.
On the one hand I would like to use a skater helmet because it provides more protection especially around the side of your head, and lower down near the base of your skull. I’m a little concerned about the extra weight especially when doing Muni and you’re being thrown around, and also the lack of ventilation. Then again I suppose it’s just down to some practice and getting used to the weight and dealing with it. As for the ventilation… again I suppose it’s just something to get used to.
Thanks for making it more confusing for me 
… (not really confused, just undecided)