I use both. Flashing lights by themselves are hard to judge distance to, but are definitely the most visible at longer distances. In some parts of the world (NL for example) they may be illegal to use. My typical setup is a dyno hub driving a decent quality fixed tail light, then something like a Radbot or PB Superflash in blink mode on the seat post. I agree with Machka that they are annoying in a group situation.

I use a flashing during daylight hours, especially if it is overcast. It is amazing how far you can see a flashing taillight.

Aaron