The difference between paypal and western union is pretty much nothing aside that paypal boasts security when there really is none. Paypal is owned by ebay and pretty much advertises absolute security when there really is none. And scammer can get a paypal account and accept credit cards using paypal.
Western union does not advertise security but will try to make sure you know who you are sending money too.
Both can be used for money transactions. From my personal experience I wouldnt trust paypal as a source to accept payments because they tried to steal 23,000 from my paypal account with no reason. And thier customer service personel are a bunch of lying robots programed to stall and delay.