Well, if it were me I'd get the 820 but it depends on what's important to you. The 521 gives you audio return channel if you need that and a bit beefier power supply (415w vs 245w). The 820 gives you built in ipod control, sirius radio compatibility, and 5-way binding posts for all but the zone 2 speakers.
There's also styling but that's on you.
Yamaha Aventage RX-A740
Martin Logan Motion 12 Mains
Martin Logan Motion 8 Center
Martin Logan Motion 2 Surrounds
SVS SB-1000 Subwoofer