Besides its obvious capacity to drive a greater continuous load, the greater watt figure provides more 'cushion' to carry it through a startup surge.
The lesser watt figure is easier to troubleshoot if you were to build it yourself. Easier to eliminate spikes, etc. This is merely a generalization, not a guarantee.
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If your load is always less than 250W, then the 250W converter is adequate. It may even be more efficient than a larger converter. (One more generalization, not a guarantee).