970a ud 3 vs 990xa ud3

Solution
Newer chipsets often work with other newer devices an older chipset might not, often work more efficiently with devices (drives, ethernet, USb, PCI-E, etc), quite often better power management for OCing, have additional ports added, better DRAM management, etc

Tradesman1

Legenda in Aeternum
Newer chipsets often work with other newer devices an older chipset might not, often work more efficiently with devices (drives, ethernet, USb, PCI-E, etc), quite often better power management for OCing, have additional ports added, better DRAM management, etc
 
Solution
like tradesman1 say's the 990 fx is a full feature chipset then a 970 and more enthusiast minded only limited to what the boards manufactures put on the boards

example as the ud5 is more feature rich than a ud3 - the 990fx ud3 is a poplar board and at its price today I don't see where you would go wrong with it for a nice am3+ build [also it supports 220w chips ]