SA is right that this is an "Incorrect question" - there is no "best" result for any overall class of program. Even for your example "say you're writing an inventory control program" the big question with selecting the file type is not "which is faster" but "is this application going to be multi user or single user?" If the answer is multi user, then your own files are irrelevant - you have to go with an established database system because the alternative is far too much work.
Similarly, Access Databases are relatively simple to set up and administer, but a bit cr@p when you start adding users. MsSql is harder to get up and running - it needs a running server to work - but simple to use and administer, and very very scale-able when user needs expand.
Even in the context of a single user environment, it is not a "Do this" - it depends on so many factors. For example, the fastest-system-to-code is probably a text file:
File.AppendText[
^] will work fine for a logging system in a single user environment, but is a huge PITA if you actually need to review or edit the log.
Horses and courses with everything, I'm afraid!
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