Platinum use is pretty heavily overweight in industrial areas; according to the linked Wikipedia article, 239 tonnes of platinum was sold in 2006, of which 130 tonnes went to vehicles emissions control devices and another 13.3 tonnes to electronics. Gold sees substantial use as an investment as well as to hedge against economical decline and inflation, with comparatively little industrial ("real world", as some put it) use. That is their principal difference from an investment point of view.
According to Wikipedia's article on platinum,
... during periods of economic uncertainty, the price of platinum tends to decrease due to reduced industrial demand, falling below the price of gold. Gold prices are more stable in slow economic times, as gold is considered a safe haven and gold demand is not driven by industrial uses.
If your investment scenario is a tanking world economy, for reason of its large industrial usage, I for one would not count on platinum to not fall in price. Of course gold may fall in price as well, but since it is not primarily an industrial use commodity, I would personally expect gold to do better in such a scenario.