There's nothing wrong with a 7970; it's just an older model.

Highlights that come to mind:

-7970 is a 802.3af class 2 device; 7975 is class 3.

-7970 has a 10bit color display; 7975 has a full 16bit display.

-7970 does not have gigabit Ethernet unless it is a 7971; 7975 is all gigabit.

-7970 does not include a wideband handset or speakerphone; 7975 does. This relates to the G.722 codec.

-7970 has a four-way rocker; 7975 has a five-way D-pad for navigation

-7970 does not support hook switch control (HHC); 7975 does.

There are probably others. The data sheets should articulate the differences pretty well.