Both of those receivers are going to be close. I would give the nod to the Denon 3803 however for resolution of soundstage. I've heard both in different systems however. But I don't think you'd lose with either.
Thats a very nice speaker system you have there, I would say very,very nice would not be to strong a statement. I would get something a lot or at least higher up in the chain than what you are thinking of. I know you might not like that thought ($$$) but thats what you get when you buy very good speakers. I would try to save a little more money and get something better. If you have to have something to get you buy for a while then buy something used (cheap) that you can resell and get your money back on untill you get more cash. IMHO.
I've compared both units side-by-side in two locations. I decided against both (for different reasons). The Denon sounds grainy when placed directly against the smooth and musical Marantz 7300. The Onkyo, while better than the Denon, is cheaply made and not as musical as the Marantz. I can't figure out why Denon is such major name in home theater.