Technically, there's no big difference. Economically speaking, it's better to use the smaller transformers.
The difference in price between, for instance, five 50W transformers and one 250W transformer is neglectible. The five 50W versions might be slightly more expensive.
The big difference comes when a transformer would start to malfunction. When one of the 50W transformers goes to pieces, the four others (and their light sources) will keep functioning and you'll only have to replace one cheap transformer.
When the 250W transformers falls out, you don't have any light left and you have to replace an expensive transformer.
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