Depends upon the instrument, which flute, which whistle! There are many kinds of whistles! The most common ones in Ireland tend to sound more "air-y." These do not seem quite as loud. The key is to blend in with the ensemble with which one is performing, and to trade off the lead. I have played with some fiddle players who drown out bagpipes! I have never drowned out a piper with a whistle.
   There is a total & complete difference between the whistle (feadog) and a flute. There are differences between flutes. I have some bamboo flutes which are not at all loud & vice versa. Same with wooden flutes. If by silver flute you are referring to the flute as in an orchestra, yes, the volume can be controlled by an experienced musician. Some flutes are of different qualities and the upper register will stick out more. Out of tune higer pitched instruments will stick out horribly.
    Instruments which have higher pitches will always seem to stick out more. Also, many American whistle players tend to prefer whistles which have a solid, flute-like sound and seem louder. I have whistles which are at varying degrees of volume. The higher the pitch, the louder it will sound. Some whistles are in high keys which I will only play outside-literally.