I have been comparing the unit and have noticed the following differences right off:

990 Technical Advantages:

7 video inputs (only 5 for the 970)
Dual mono sub analog preouts
USB port
Auto Setup
Lip synch in 1ms increments (10ms increment for 970)
Assignable DVI inputs
Zone 2 capability
Balanced Preouts
Theater Compensation
Assignable Component Inputs
Better DAC in front channels
2 Trigger Outputs (970 only has one)
Phono Input
2-channel Upsampling


970 Technical Advantages:

Fixed Analog Bass Management
Auto Video input selection
4 Coax inputs (990 only has 2)
Individual input level trims
Auto-Polling
Bass/Treble settings per individual video inputs
6.1 or 7.1 capability
Better range of OSD timeout settings
Setup via Front Panel Display


Feel free to correct me if I am wrong on anything.