How will a bitcoin ETF be any different from a bitcoin exchange?
You don't get the bitcoins themselves.  Your investment fluctuates in response to the exchange rate.  From the brothers W:


  The investment objective of the Trust is for the Shares to reflect the
  *performance* of a weighted average price of Bitcoins, less the Trust’s
  expenses.


In other words, if the average exchange rate USD/BTC is $230, and you buy $230 worth of the ETF, when the average rate goes to $250, you could sell the ETF shares for around $250 ("less the Trust's expenses").  All of your transactions would have taken place on a US stock exchange, using US dollars.

You could do this yourself, by buying and selling BTC on a variety of exchanges, but your profits would be more than eaten up with the commissions for buying and selling.  

The ETF, on the other hand, would presumably have the benefit of volume, and may not even have to invest all incoming funds when there are outgoing funds (from redemptions) at the same time.