NPO (Negative-Positive-Zero) refers to the capacitor's ability to avoid a change in value with temperature.
Silver mica capacitors have low resistive and inductive losses (high Q-factor), and also their properties are pretty independent from frequency.
So, silver mica capacitors are better for filter design, when both, NPO capacitors and silver mica capacitors are good for oscillator design.
Also, due to their lower losses silver mica are best suited for high power applications.
I don't know your requirement, but in RX where the power is not an issue probably you can use NP0 capacitors, saving money.