Aluminum or Plastic for a board with PCB trace antenna
To the extent that your aluminium box doesn't have large holes (relative to wavelength) in it, it will act as a Faraday cage and significantly attenuate any electromagnetic radiation passing between the inside and the outside.

Sometimes, this is what you want. If you are trying to reduce unintentional emissions of your circuit, or reduce noise received from the outside, this is good.

Sometimes, this is not what you want. If your PCB includes an antenna, you probably want to radiate electromagnetic energy, so enclosing it in a conductive box is probably counterproductive.