I thought 900MHz or 400MHz would travel farther than the 2.4GHz that the nRF24L01 runs on. This application isn't transferring a lot of data, so the longer wavelength wireless of the RFM69HW might be better than the nRF24L01. With the RFM69HW, you can select different frequencies (415MHz or 915MHz) during configuration, so you can try out the different frequencies to see which might work better.
The RFM69HW is only $4 per transceiver. Just a bit more expensive than the nRF24L01. It's newer than the nRF24L01, so there's not as many examples of people using it. But it seems like a very capable wireless comm module.