The comparison of these two models is not quite a fair one as the 2809 is no where near as equiped as the SR876. A closer comparison would be between the 3808ci and the SR876. That being said, the 876 will stomp on the 2809 in terms of power and they both have all the new Audyssey technology...and as a plus the 876 has Reon on-board as well....
If these are your two choices, go with the SR876--just make sure you keep it in an open well ventilated area as these AVRs get very hot....not closed cabinet for that thing. Now if it were between the 876 and the 3808---I would take the 3808

Edit: As for your last question as to reputation, the Denon definitely has a more clean record for reliability...as for sound quality---thats very debatable----Onkyo owners here on Audioholics are very happy with their Onkyos; just as Denon owners are very happy with their Denons...