As Diodorus Siculus correctly says, Flash Memory is a name which covers all of the storage devices such as SD, xD, CF, SMC, etc.
Or are you referring to Compact Flash as Flash? Generally when you buy a camera, the features of the camera would be the most important, however, if you already own a certain type of media then it would probably be preferable to favour a camera which uses the same type of memory because it will save costs of buying more memory.
If you have no preference, things you may wish to consider between different storage mediums are the cost, capacity, transfer rate, size. At the moment, SD seems quite popular for newer compact digital cameras. High end cameras still tend to use Compact Flash.
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