When to use P-Type current source(using pMOS) and when to use N-type current Source(nMOS)?
For a current source the small signal impedance provided by it is ideally infinite. In case of transistor implementations, it is crucial which terminal we are "looking into" when we are defining the small signal impedance of the current source. If we use an nMOS between Vdd, and a node to which another MOS perhaps is connected, we would be measuring the small signal impedance "looking into" the source, which gives an impedance of 1/gm, quite low, thus deviating from the actual nature of a current source. Whereas if we use a pMOS to implement it, we would effectively be "looking into" the drain, giving an impedance of ro (due to the channel length modulation, or infinite if it is excluded) which is quite a high value. You can use similar observations to ascertain the behaviour between a node and ground and which MOS to use.
Hope it helped.