Precedence of Mitzvot

A frequently performed commandment generally takes precedence over a less frequently performed commandment, but ONLY:
  • Regarding the order in which they are to be performed, and
  • If there is no specific reason to do the less frequent one.
If you are only able to do one of several commandments, do the most important one. 
Example If you can only put on either talit or tefilin, you would put on the tefilin since that is the more important commandment, even though putting on a talit is the more frequently performed one.
Another Example Friday before sunset when Chanuka will be on Shabbat--lighting Shabbat candles is done more frequently, but we light the Chanuka candles first since if we lit the Shabbat candles first, it would already be Shabbat and we could not light the Chanuka candles at all.