They are two very different receivers. Both of which I have been able to review.
Sony STR-DA2800ES A/V Receiver Review | Audioholics
The 2800ES has some pretty cool features, like Control4 integration, Netflix, and a 4-port switch. But it lacks multichannel pre-outs, has mirrored sub control, and a cruddy auto setup system.
The X4000 is much more audio oriented. It has 11.2 pre-outs, dual independent sub control, a much better auto setup system, and larger capacitors. It also costs $300 more.
The X4000 is definitely the better choice for those concerned with the best sound quality. The 2800ES is for a different type of consumer.