Hi difference is max lamp loads, go for lowest that will be more than you need! ie if dimming 1 X 100w lamp then get a 250W dimmer as thats the smallest made, if say 3x 100W lamps you need next size up, ie 400W sw.
If you 'play safe' & just buy a 400w sw. you will find if only 100w loaded it won't dim low enough,
P>S> these ratings are for normal tungsten lamps, if using halogens then allow 50% more rating (approx) for cold inrush current, also if using Low Volt fittings get a dimmer for transformer rating.
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