The Sonicor S-300 has a unique form factor for a tabletop ultrasonic unit of it's size. The S-300's "Tall" tank has more verticle room compared to the similarly sized S-311, making it ideal for situations where height is more important than depth.
The S-300 comes in four sub-models:
S-300 - Standard unit, simple on/off switch for sonics
S-300T - Includes mechanical timer, 1-60 minutes.
S-300H - Includes in-tank heater, simple on/off switch for sonics
S-300TH - Includes mechanical timer and in-tank heater.