I always wanted to do one of my ads in the Boston Globe with a picture of a platinum ring and the headline: The Other White Meat...Metal (with Meat with a line through it). In fact it isn't the other white metal, as it really is a white metal as opposed to white gold that is actually yellow metal posing as something it isn't.
Platinum is normally worked as either a 900/100 platinum/iridium mix (platinum is almost always alloyed with a member of the platinum metals group) or a 950/50 platinum/ruthenium mix. The numbers refer to the parts of platinum relative to the parts of alloy. There are other alloys available and recently some companies have started to work with a 585 mix (14k yellow gold is 58.5% pure gold) although they aren't allowed to call it platinum as the metal quality laws don't allow anything less than 900/100 to be called platinum. Personally I like that platinum is used with such a high purity level (I only sell 18k gold or higher for the same reason) and it seems foolish to cheapen it so much. but there is always someone out there looking for a way to make a buck.
Platinum is a more expensive metal than gold for a variety of reasons. One is that it generally is more per ounce than gold is (although they are getting incredibly close in price at the moment as gold continues to rise due to the value of the dollar falling so much). It is also worked purer than gold (90/10 or 95/5 as opposed to 75/25 for 18k gold) normally. It is also a much denser metal so the exact same piece in platinum weighs far more than gold does. And to top it all off it is a much more difficult metal to work with. While hand constructing platinum is certainly possible, casting is a far easier option normally. Platinum is soldered or fused at such high temperatures that protective eye gear is necessary whenever working with it and it takes far longer to attain a suitable finish than gold.
There is some confusion about the durability of platinum. Depending on who you talk to you'll hear that platinum scratches much easier, that it is harder than gold, that it lasts longer, doesn't hold up as well, you name it. In fact all metals scratch, especially when used in rings. Platinum does, in some cases, seem to scratch a little easier but it builds up a nice patina as it does. However, the difference between platinum and gold is that when you scratch a gold ring you actually remove metal from it. When you scratch a platinum ring most (although not all) of the metal is simply moved from one point on the ring to another. Hence it is actually a more durable metal as it will wear away more slowly.
So the question becomes should you get platinum or should you get white gold? In my belief, you should get the one which has the color that pleases you the most. This is a little tricky when buying commercially made rhodium plated white gold as it starts out looking almost the same as platinum, but you do have to remember that it will change in time. My 18k palladium white gold has a different appearance altogether than platinum. There is nothing wrong with it. It is just different.
Personally I believe white metals were put on Earth only to accent yellow ones and anyone asking me would be told, get a yellow gold piece. But I'm not the one who's going to wear it. For those of you who love white metals, get the one who's color you like and enjoy!
Pictured above is one of my platinum rings with a color shift purple/blue sapphire and diamonds.