Semiquaver rest and Quaver Rest Reg.

The answer to your question is a question. On what beat is the rest? If the quaver rest is on a quaver beat, then use a quaver. If the quaver rest ends one quaver and starts another, use 2 semiquavers. It is easier to show using crochets in various times like this.

There are 2 identical bars except for the time signature. In the 3/4 bar the crotchet rest is the entire 2nd beat. In the second bar the crotchet ends the first beat and starts the second beat, 2 quavers are to be expected. The same idea can be extended to any other rest and time frame.

This is the normal rest pattern and retains the rhythm of the music and helps the musician to keep the music lined up with the beats. You will see this courtesy violated continually in songs, even in the most respected of music printers.