Flashing red is better for getting attention but it isn't as good IMO for gauging position or heading as a solid light.
I had a bit of a close call once with a cyclist whose only rear illumination was a red light flashing at one second intervals and the first moment they would have been noticeable to me also coincided with the light being out which due to them wearing dark clothing, etc. delayed me spotting them by a whole second - if they hadn't had been wearing white socks there is a good chance I'd have not noticed them until too late.