It should handling blinking down to at least 4.0 Hz, and preferably down to 0.5 Hz. It should handle blinking up to at least 18 Hz, and preferably 25 Hz or more. 2 LEDs per eye or fewer is one of the marks of an inferior machine. Look for a good, easy brightness control, too. This is one of the major points of difference between the various manufacturers.
Most machines use red LEDs, because they're cheap, bright, and low in power.
Synetic uses amber LEDs, presumably because they're brighter. Photosonix offers you your choice of any of the above, and even has units with multi-colored LEDs so that you can pick the color as you program the session. However, none of the manufacturers has offered any significant proof that it matters , except as a matter of personal taste. To do that, a light and sound mind machine must have some kind of timer, and an ability to specify starting and ending frequencies. The tones will play either continuously or at the same time that the lights are lit.