Both the 2805 and 3805 are excellent units. Yes, it's true that the 3805 does not have A-B front speakers. What the 3805 has instead are A-B side surrounds so you could use dipoles/bipoles when watching movies and monopoles for multi-channel music.
And as a clarification, what the 3805 has over a 2805 is:
20 more watts per channel
8 pounds heavier
HDCD decoding
fully illuminated electroluminescent remote with motion sensor
Burr-Brown DAC's
Denon Link (for lossless SACD/DVD-Audio)
flip panel front (fewer exposed buttons)
And yes, you can easily get the serial codes, etc. from the Denon website which I have already provided a link to.
Are these features worth the price difference over a 2805? I'll let prospective buyers, each with their specific budget, to decide. But you will not be disappointed with either unit.