Accutronics® Type 9 reverb units are available in three different decay times: short (~2 seconds), medium (~3 seconds) and long (~4 seconds). A CoolSprings system can be configured with any combination of decays, but typically, you will want a purely short, medium or long decay unit.
According to Accutronics, the short decay is best for vocals and drums, medium decay is best for organs and other keyboards and long decay is what is normally found in guitar amps. So, of course, you should buy one of each...
Combinations are also possible: short-medium, medium-long and short-long. These are not really in-between decay times; rather, they are a superposition of two reverb units with different decay times.
The modular nature of the CoolSprings design allows reverb units to be swapped easily. If you purchase a unit and then decide that you would rather have a different decay time, the reverb units inside can be exchanged for new ones.