I have the Sabertooth 990FX in my computer and I really like it. I'd say it's the better of the two boards you listed.
The reason for the prices probably has something to with the value of the components, and ASUS marketing strategy.
Probably the most significant difference between those two boards is the AM3+ CPU socket on the Sabertooth 990FX. That means that it can support AMD FX series processors whereas the M4A89TD Pro/USB3 cannot.
Some other differences: Sabertooth supports AMD CrossfireX and Nvidia SLI multi-GPU's. M4A89TD Pro/USB3 supports AMD CrossfireX but NOT Nvidia SLI. This is because AMD chose not to enable SLI support on the 890FX Chipset but changed their stand when they released the 990FX chipset used on the...