Marantz 8nnn series equipment generally has higher quality electronic and mechanical attributes than 7nnn series equipment. E.g. gold plating on the connectors as well as some analog circuit differences. The AV8003 has s-video connections which the 7nnn systems lack. Also, the AV8003s were made in Japan, while the 7nnns have been made in China, which matters to some people.
However, the AV7005 and AV7701 models provide a higher grade of Audyssey room correction (MultEQ XT) than is in the AV8003, which has just MultEQ. They also can apply Audyssey and decode the HD audio formats simultaneously. The AV8003 has a less powerful DSP, so you can either bitstream the HD audio formats from a BD player and not use Audyssey, or you can decode HD audio formats to PCM in the player and apply Audyssey to the decoded audio.