TIP: Don't forget to bargain with the Europcar guys!
Long story short, I spoke to a helpful rep in the booking office, told her what other companies were offering and asked her to match it etc.
I ended up with a large family car (F class) with kids seat included (1) picked up and returned to Brissy airport (premium location charge etc etc) for $276 for 5+ days - total. I think she gave us a free upgrade and a free day (Thrifty was doing a 2-class upgrade I think) and she also gave us some leeway in returning the car late (over an hour; normally they charge a third of the daily rate).
Possibly this price looks expensive to non-family types, but it's a huge saving on their "rack rate", especially when you consider the extra charges for airport pick up and return etc.
I recommend locating your deals, proceeding with your booking online, and getting all your info before you call the company directly - voice-to-voice definitely gets a better result, and I see no reason why it wouldn't apply equally to any other company if you get better points elswhere etc.