For me a bullion's a silver (or gold) coin made from .999 & more. We can invest in bullions and we can collect them also. When you want to sell them to a dealer, he'll give you spot for generic and spot + 1 or 2 $ for Govt bullion. When you want to sell them to a collector you will sell them for the perceptional price (Mintage and demand). I collect, invest and sell 1 oz rounds and bars. When I deal with a stacker he will not pay me more than spot + 4 for a wolf or ASE or koala. But when i sell to a collector a wolf or Kookaburra he pays for the price even if it's spot +10 $ because he knows that the perceptional price of the market is spot + 15 $ for example. For me all these MS/PF is marketing. It's like paying the same orange on a fruit store for a price and paying the same one at 5 stars hotel at price * 5 !!!
All bullion coins can be collector coins but not all collector coins are bullion coins. Of course, this statement could be challenged on the definition of bullion. I'm thinking that gold and silver are bullion, for instance, and copper, zinc, and stainless steel are not.
Side note: That would not apply to the euro area where the term "collector coin" has a specific meaning. http://ec.europa.eu/economy_finance/euro/cash/coins/pdf/types_coins_en.pdf Of course we may collect whatever we want ... Christian