I'd go with a P7 flashlight myself. in my experience, dawn/dusk is when you need MORE light because you may be riding into the sun. I want a really, really strong taillight at those times too.

You can get a P7 flashlight with mount, batteries and charger for $50 to $80 and they're really quite bright. At dawn/dusk you will probably want the option of a strobe.

There are also fairly powerful smaller lights that take AA cells, I'm sure someone will point them out, and also my advice for "P7" flashlights is likely out of date, there are probably brighter and/or cheaper LEDs on the market now, I bought my P7 a year ago (I use it as a backup to my MagicShine). I use my MagicShine daily but I think the P7 is probably easier to deal with. The MS works for me because I also have their taillight and an aftermarket high capacity battery. But you can't beat the simplicity of a nice bright flashlight.