State Street's started trading first and has managed to capture a larger slice of the market. In February, the GLD ETF held $6 billion worth of assets. However, both should be equivalent bets for those looking to invest in gold. In its first three days of trading, GLD traded roughly 30 million shares and nearly all of that has been new buyers if you believe the press.
I, for one, am not interested in holding too much of this particular asset class. Why? Because gold has no real use in the world. Sure, it is admired and hoarded by people across the globe, but it doesn't generate value on its own. Warren Buffet said it best, "I would rather own assets that produce value. Dow went from 66 to 12000 and paid dividends. If you owned gold you paid 20 and went to 400 a hundred years later."
However some investors are attracted since gold is likely to increase in value when other areas of the market are suffering. As such, it is used as a hedge against other investments. Regardless, I prefer to invest in the long-term returns that company stocks and bonds offer. Which isn't to say that I don't own any gold. In fact, the commodities ETF that I have is 10% gold.
Note that gains from the gold ETF will be taxed at the collectibles rate of 28% vs. the long-term capital gains rate of 15%. If you're going to invest in this ETF, you might want to consider using a tax deferred account. And since gold doesn't produce income, partial shares of your holdings will be sold to pay for management fees.