db_to_client first reads the blob and sends it in junks to the client; wpg_docload doesn't do that. So if network roundtrips are a concern of yours then wpg_docload might be a bit faster.
If you don't connect directly to the database via JDBC in your custom JavaBean you'll mostly end up rewriting the same functionality as webutil_file.db_to_client, so unless you are using JDBC (which might introduce a load of other problems) I don't think this is what you want to do. And I wouldn't connect via JDBC from the bean either.
IMO performance wise there shouldn't be much difference between wpg_docload and webutil (although you might test that assertion of mine), so I guess it really boils down to what you like more. The document table feature forces you to jam various objects in one single table whereas webutil doesn't require a specific structure of the table containing your blob; However webutil can be only used from within forms whereas you can display / transfer your blob very easily from any tool which is capable of doing HTTP Requests.
I for one would do it via wpg_docload, but mostly because we don't use webutil plus we have a strong possibility that I can (and probably will have to) make use of the feature in another tool as well.