I dont have exactly the same player or TV as you but I have a very simular set up so hope I can help a bit.
I have the Denon3930 which replaced yours and a Samsung PS50P96FD which is 1 generation behind your screen but also Full HD. I have found the best results come from using the AUTO option on the Denon (admitidly mine picks 1080p which is not a option for you) but the player does select RGB over y cb cr, tho I am ******** if I can see sny differnce when swutching between the 2.
As for setting your 3910 at either 720p or 1080i my advice would be to try both settings with a DVD you know well and se which you prefer as you may find 720p offers a smoother picture letting the screen upscale to 1080p, or set at 1080i and the screen only has to de-interlace the image with no need for extra scaling as to be honest the scaling in you Denon is better than that in the Samsung.