Denon fans will tell you that the 3803 is better. Yamaha fans will say the 2400 is better. I doubt you'll ever get the answer you're looking for. The 3803 has a tiny bit more power with all channels driven. It is extremely doubtful such a small difference will improve sound quality. Besides, it's rare that all channels are driven consistently at maximum power for an extended period of time in any soundtrack, in which case the power outputs are about the same. I'd say if the Yamaha isn't enough power, you'll want a lot more power than the Denon too.

I think "separation" has more to do with speaker setup and receiver settings 99% of the time than the actual sound quality of the receivers.
Best to test them for yourself find out which you prefer.