I have had the Onkyo 808 for about a little over 6 months now and love it! Seems to be somewhat of a "buzz" latley about Onkyo as they have a few issues from what I have read but I can say its been a solid performer for me. The809 is going to have a much larger cabinet design & weight vs the 609... so you may want to take that into fact when you deceide on placing the unit.
The 809 has the much better Aud. set up as it has the Audyssey MultEQ XT vs the Audyssey 2EQ the 609 has. The 809 has a bit more power, about 35w more per "2" channel but I'm not sure if that would be a huge factor for you or not. There are a few differance's but it just depends on your needs is all... as to if the "extra" dollars is worth it for you or not. I doubt you may need it but the 809 is bi-ampable to where the 609 is not, more than likely you wont need it but I thought I would at least list it in case you were thinking about it. Depending on your speakers, the 809 is also handle 4ohm speakers as to the 609 looks like it can not.
I dont know much about the newer 1080p Upscaling the 809 & 609 have as you may want to check into that a bit more. The 809 has the HQV Vida vs the 609 has the Marvell Qdeo.... as mentioned, cant help you on that one since my 808 does not have that but the 808 works quite well IMO.
In most cases people just put the "pass thru" on as they let the TV/Player do all the upconverting and scaling so thats something to take into fact as well. Its hard to say if the extra features are worth it or not as you will have to look into that. I believe if the extra features arnt a big deal or concern then I would amp for the 609 as it would save a few dollars... just me though.