Hi Don,
If you use Fast Reply your browser does not need to load a fresh page, so it's quicker to use.
But in Fast Reply you can't have automatic quotes, you can't have Attachments, and you can't use the Options to add a post smiley (like the yellow-arrow one I've used here).
If you start off in Fast Reply and want to change to a normal reply, click the Use Full Editor button.
regards,
Martin.