Depends on the socket - some Rampage board have additional PCIe sockets, some have additional SATAII/III ports, some have additional USB 3.0/2.0 ports, it's really easier to click the compare link between the different boards you are researching.
The main difference between the 2 is the Sabertooth boards use TUF for several of the components and on the 990FX it uses a ceramic coating on the heatsinks adjacent to the proc. They also have tested those boards a little more rigorously and they have a 5 year warranty.
I picked the Sabertooth 990FX for my build because of:
the additional 2 years of warranty
I am not planning to SLI so I don't need the additional port options
I only use an eSata 3GB drive dock, don't need an eSata 6GB
Only have 1 device currently using USB 3.0, don't need additional ports
none of the other features seemed enticing enough to spend the extra $100
If I were in your shoes (which I was), I would look at the features that are different between the devices, look at what you're intending to use in your build and figure out whether you want 2 more years of warranty or additional PCIe ports