Josh,
It would depend on the components specific role......
If I have a transformer, either power or isolation for a powersupply, I want it to have low bandwidth. I want it to pass just the 50/60hz mains power frequency and would like the higher frequencies not to get through. So all things being equal (which they never are) I would choose a split bobbln EI, R or C core transformer over a typical toroidal transformer. For a series function of passing AC voltage I want low bandwidth, just sufficient to pass the mains frequency with deminimus attenuation, and would like high attenuation of higher frequencies.
Typically, the role of a capacitor in power conditioning is that of a shunt.... to take noise on the 'Hot' line, and redirect it to the Neutral or Ground lines. [This is a gross simplification, as there is also noise on the Neutral line, etc... and the capacitor also makes differential noise into common mode noise which can then be 'dealt with via the transformer] Noise on AC lines is high freqency in nature (lets not talk about DC on AC lines which is certainly noise, but has been discussed on other threads). Remember your Fourier analysis? In this role we want the capacitor to have as high a bandwidth as possible because we want to divert it (shunt) that noise off of the 50/60hz power.
Long story short - capacitive power decoupling, whether AC of DC rail, should be high bandwith, high dV/dt, and low inductance as we want noise to pass through that cap.
