I hate to sound like a complete moron here, but can anyone tell me the difference between a sperm plug and viable sperm? Ok I know that viable sperm is "the good stuff" but is a sperm plug just old, not potent sperm? Or is it immature?

Also, I'm curious to know...for those of you that have your own rack systems/incubators, what kind of thermostat do you use, and what is the big difference between on/off thermostats and proportionals?