I found that two lights - one flashing, one fixed - was the easiest for me to see as both a fellow cyclist and motorist. I could judge the distance much better.
Flashing a bad idea except at dusk and in emergencies (dying battery). Can't judge speed of flashing lights and really annoying for other cyclists as well.
Or spend a lot and get an exposure one as these are really well made and reliable as well as obviously being very bright. Joystick seems to be on offer at the moment.
The new Exposure Trace looks great value at £50 and ticks everything but the battery life. 5 hours is probably actually enough - there's not that many weeks when I'm using morning and evening. Has anyone seen one?
However, in my experience Exposure lights have been anything but reliable. We've had two early joysticks that had circuit board failures and my current Race has never held charge properly. Used to go completely flat in about a month if not used. Went back to exposure and they claimed nothing wrong. Just got it out to start using for the winter (I've charged it a couple of times over the summer) and it now seems to die in 2 days (charged Friday day, full friday night, full yesterday afternoon, dead this morning ). A commute light needs to be more reliable than that.
Will call exposure tomorrow but their policy before has been a new light at a discount. Given how long the others have lasted they still work out expensive that way