parallel filters vs series filters
They are not both low pass filters. The upper circuit is a high-pass filter.

But the fact the capacitor is labelled \$\rm C_f\$ suggests this filter might be used in the feed back path of an op-amp circuit. 

In that case, the overall op-amp circuit might very well be a low-pass filter. Because using a high-pass filter to increase the negative feedback at high frequencies will result in reducing the response of the overall op-amp circuit at those frequencies.