I've used all three. Nat, Com and Westpac. I still use them but mainly use Westpac. I have a good deal on brokerage fees with Westpac but have troubles with some trades being delayed (referred) when I want to get a buy and a sell on the same share in a hurry, especially in preopen bids and at close of trade bidding. In my opinion I think I get the best service from Westpac.
