Difference between 990fx and 970?

Solution
970
Codenamed RX980
One physical PCIe 2.0 ×16 slot, one PCIe 2.0 ×4 slot and three PCIe 2.0 ×1 slots, the chipset provides a total of 22 PCIe 2.0 lanes and 4 PCIe 2.0 for A-Link Express III solely in the Northbridge
HyperTransport 3.0 up to 2400 MHz and PCI Express 2.0
13.6 Watt TDP
Southbridge: SB950/SB920

990FX - Latest one.
Codenamed RD990
Four physical PCIe 2.0 ×16 slots @ x8 electrical which can be combined to create two PCIe 2.0 ×16 slots @ x16 electrical, one PCIe 2.0 ×4 slot and two PCIe 2.0 ×1 slots, the chipset provides a total of 38 PCIe 2.0 lanes and 4 PCIe 2.0 for A-Link Express III solely in the Northbridge
HyperTransport 3.0 up to 2600 MHz and PCI Express 2.0
ATI CrossFireX supporting up to four graphics cards
19.6 Watt...
Plus the 990FX is going to have a better chance of supporting the next gen CPU's from AMD, so there is that to consider. It's possible the 970 based chipset might support the next gen too, but you are getting a better change by going with the 990FX chipset over the 970 chipset, if you think you might want the latest CPU from AMD in the future.
 
970
Codenamed RX980
One physical PCIe 2.0 ×16 slot, one PCIe 2.0 ×4 slot and three PCIe 2.0 ×1 slots, the chipset provides a total of 22 PCIe 2.0 lanes and 4 PCIe 2.0 for A-Link Express III solely in the Northbridge
HyperTransport 3.0 up to 2400 MHz and PCI Express 2.0
13.6 Watt TDP
Southbridge: SB950/SB920

990FX - Latest one.
Codenamed RD990
Four physical PCIe 2.0 ×16 slots @ x8 electrical which can be combined to create two PCIe 2.0 ×16 slots @ x16 electrical, one PCIe 2.0 ×4 slot and two PCIe 2.0 ×1 slots, the chipset provides a total of 38 PCIe 2.0 lanes and 4 PCIe 2.0 for A-Link Express III solely in the Northbridge
HyperTransport 3.0 up to 2600 MHz and PCI Express 2.0
ATI CrossFireX supporting up to four graphics cards
19.6 Watt TDP
Southbridge: SB950
Enthusiast discrete multi-graphics segment
 
Solution
Most 990FX boards support SLI while 970 ones don't, few exceptions would be the Asrock 970 E4 mentioned in this thread. As for specific differences between the two board, you can easily use Asrock's product pages. The 990fx E4 is better for OC-ing than the 970 E4, that's obvious. You should also check the Asus M5A99FX Pro 2.0 and Gigabyte 990fxa-ud3, which are very strong candidates with their prices sit between the Asrock 970 E4 and 990 E4. They are both mentioned here :

http://www.tomshardware.com/reviews/990fx-motherboard-review-amd-fx,3464.html