I use the dynamo version of the fork-mounted Spanninga light. It also comes as a battery-operated light. It is not especially bright, but it seems sufficient for a "be seen" light.

Crown-mounted is nice because it doesn't come off without tools, so it's less likely to wander off when you're not looking. It can also be a pain for exactly the same reason. Also it's not as versatile. I got a front, platform rack, and want to relocate the light elsewhere, but it's tricky to use a crown-mounted light anywhere else. Those adapters which let you use a handlebar mounted light will leave you with more mounting options. Also a light that slides out of the bracket, as most handlebar lights can, can easily be removed for folding.

But if you're sure you want a crown-mounted light, go for it. I liked mine up until I got my front rack. Here's another option:
http://www.amazon.com/Nirve-Vintage-...dp/B000QFRSOK/
Again, not great for light, but meets the battery and crown-mounted criteria.