Regarding the SMD resistor antenna, one caution is that section 3.3.7 High Power Settings of the datasheet says, "The Duty Cycle of transmission at +20dBm is limited to 1%, with a maximum VSWR of 3:1 at antenna port, over the standard operating range [-40;+85°C]. For any other operating condition, contact your Semtech representative."
[Edit: Also, looking now at the RFM69 library I notice that the transmit power settings are fully maxed out by default if using an RFM69HW. ]