From what I can see, they do different things. SmartReflex changes the voltage while CPU Frequency Scaling (obviously) changes only the frequency. The power consumption is quadratic in voltage P = V^2/R while linear in frequency, so there is usually more to gain by lowering the voltage. I have no idea about the OMAP processor specifically or whether my argument holds in this specific case.

At any rate, please post here or to the bug if you encounter any instability, but from what I can see, enabling SmartReflex seems to buy a factor of 3-4 on battery life.