If you want greater coverage, it is better to add multiple access points. Put one on the inside of each outer wall, preferably near a window, as high in the air as possible.
When setting up wireless access, you need to cater to the weakest client. It does no good to add high gain antennas, that will let you be seen at long distances, if your client lacks to ability to transmit a signal back to the source.
You also need to be mindful of the neighboring wireless setups. It's possible that there are enough active networks that your range will be limited by the interference.