From looking at the comparison chart at Onkyo USA, the two significant differences that stand out to me are more sophisticated video processing and 2 HDMI outputs on the 809 (apart from the power difference--not great enough to matter on spec, though the amp design and power supply seem more robust on the 809).
For me, the choice is clear because of the 2 HDMI outputs. The video processing side may be of benefit (depends on a lot of factors) but I would want 2 HDMI outputs to avoid relying on the switchbox I use now. My switchbox is fine because it is the only thing in the path between the various HDMI sources and my 2 displays. However, adding a second repeater (a receiver) between the sources and displays seems like I'd be asking for trouble, given HDMI handshake issues.
Either one will likely run pretty warmly, so make sure you have sufficient ventilation in your cabinet.
If I had only one display, I'd likely opt for the 709. But with a PJ and a monitor, I'd go with the 809 (as it is 200$--were it 500$, I'd take a chance on my switchbox or go with another AVR with 2 HMDI outputs).
Edit: The comparison chart shows only one HDMI output for the 709 but the rear photo shows 2. If the photo is correct (not always the case on websites), then my own choice would become the 709. It seems a visual inspection of a unit is in order.
Edit again: Just checked the manual. Seems like the 709 does have 2 HDMI outputs.