The difference is primarily connectivity, with the Q8 being a GSM device with EDGE capability and the Q9 packing HSDPA if you need faster web and email access. Both feature a full QWERTY keyboard, a two megapixel camera, media player and the Windows Mobile 6.0 operating system. No Wi-Fi though.
They also come with “My Q Paks”, free bundles that allow you to personalise your device, depending on how you intend to use it. Both are expected to be on sale over the summer (June/July), with pricing dependent on your choice of contract.