The purpose of the reservoir is to provide a low impedance source of current for handling pulse changes. The main power source and regulator should be able to respond to slower changes of the order of a few 10s of uS. This can be degraded by any lead inductance between the regulator and the module and is why the capacitor is best positioned close to the module.
I normally use tantulum capacitors for this. They are a bit more expensive than aluminium but are smaller, have much lower esr and this is maintained at high frequencies. One has to be a little careful to ensure they cannot be reverse connected as that can rapidly destroy them but that is not normally a problem in a simple reservoir application.