magnetic/electronic/pulse start/probe are all different types of ballasts. magnetic type ballasts tend to "over drive" some bulbs while electronic seem to "underdrive" some bulbs.. this is of course has expections as they dont always over drive or under drive bulbs as every bulb has different specs that they need to run at.
in my experience a single 150W will cover a normal size 20gallon just fine as for heat if you went with the 250W you can always lift it higher thus end up with less heat inside the tank itself, however as whatcaneyedo it also depends on where the tank is setup and the ambient temp.
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