For a link, you mainly need signal-to-noise ratio (SNR). With high gain antennas you simultaneously increase mutual reception and decrease reception of other s

For a link, you mainly need signal-to-noise ratio (SNR). With high gain antennas you simultaneously increase mutual reception and decrease reception of other signals, giving a triple boost to your signal-to-noise ratio if you can use it on both sides (sender: +1 txpower; receiver: +1 signal reception, +1 interference rejection), so that is what you should aim for. Transmit power does comparatively little (sender only: +1 txpower, -? amplifier noise), while blanketing the area with useless power (both in-band and out-of-band²) that can get you detected and affect other devices. Especially if you driver you transmitter near the limit (or use a shitty amplifier, broadband antenna) further increasing out-of-band transmissions, which is the part that is likely to get you a visit from regulators. ¹ gain ~ directionality; symmetric in both reception and transmission ² out-of-band - frequencies other than intended ones