Parsley: flat-leaf or curly?
Flat-leaf (also called Italian Parsley) and curly parsley can be used interchangeably but most chefs prefer flat leaf as it usually has a more distinct taste.  That is going to be up to you and your preference.  

Curly parsley provides a more unique and visually interesting look when you're talking about garnishes but putting a sprig of parsley on a plate is what would be called a "non-functional" garnish.  Aside from adding a bit of green it serves no purpose as very few people I know will pop it in their mouth and eat it.  You're better to sprinkle with chopped parsley or herbs that can help enhance/accent the flavor.

Curly parsley is also visually appealing used in a parsley salad due again to the ruffled edges.

I usually use flat-leaf if available, then curly if flat-leaf isn't available.  Under no circumstances (except if you just need a touch of green dust) do I recommend using dried parsley.  Clippings from the underside of yor lawn-mower probably have more flavor.  Same goes for dried chives.