Never Flash. IMO

Apart from the bugs in the Windows implementation that don't seem to show up in Mac, it's a great big security hole that you have little control over. Updates for Windows seem to trail behind Mac and

Many people will by default block flash completely, and only allow trusted sites to run it.

Flash has the advantage that it's harder for people to steal your images. But also harder for any search engines to index your site for them to find your images.

Commercially all we use it for at work is adverts.
I suppose that says it all, really.