Buy the decent one (pure sine). Two reasons.
1. Square wave or "modified sine" ones generally have a high power due to the shape of the output wave form. This is not the kind of power that is useful as it only appears as heat in copper components... Most older psu's can cope they have a transformer that ineffect smoothes this out to "more" of a true sine. Most newer psu's are switched mode. The chinese are very good at producing cheap (few component) switched mode psu's. These also do not like dirty mains. Better designs generally cope much better.
2. Cheap inverters have cost savings built in. best place to save cost is in copper. This means under specified output components, and can result in fire.
But having said that, I have a S**ty 150 watt inverter by ring. My netbook psu stops working every now and again. One day I will buy a proper one.
Hope that helps a little.
Dan
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