Hi,
Just reading this thread and thought I'd mention something.
I've got the Weber Family Q at home, amazing piece of kit. It to has the thermometer in the hood, which I think works really well. Awesome roast porks !
If you use one of the after market thermo mentioned above, you need to lift the hood to check the temp, thus lossing valuable heat. This inturns effect the cooking of the item in the bbq. If you read your booklet that comes with the Weber Q series it strongly suggests against this.
You can purchase a remote thermo which you stick into the cooking item and place the receiver some close to where you are, upto about 10 metres I think. This maybe a better solution.
I'd go with the model that has the hood thermometer.
Cheers, Scott