From a clinical / experimental standpoint there's very little difference between 4,8,16 hours. It's all essentially 'next day' right now because common practice is to send the sample off to a genomics core and they drop the data on a server for you after they've run it through their pipeline. Many experiments require their own library construction but clinical pipelines generally don't and again, the machine prep time is not the bottleneck it's deciding what to include and how to lay things out. From what I understand 454 accommodates the longest read lengths right now but this seems to change month to month, also from what I understand 454 has a lot of trouble with short length libraries.