Stainless steel is used primarily in English bits because it doesn't rust and is widely available. It's considered a "neutral" metal. It has a little taste and does nothing to encourage salivation. The mouthpieces are stainless steel mouthpieces are with copper inlay. Copper also encourages salivation to keep the mouth comfortable and soft.
Western bits and the Combination bits are usually made of Sweet Iron with copper inlay. Sweet Iron is a strong and durable metal that actually starts to oxidize when exposed to moisture like saliva and releasing a sweet taste in to the horse's mouth. A sweet iron mouthpiece will change colour and turn from silver to dark gray after a few uses. Sweet iron encourages salivation, this is desirable because it keeps the mouth comfortable and soft.