Hello David, thank you!
I’ve always loved GNU radio and have been using it for many years, starting with a USRP1 and two basic tx and rx boards... the software has improved a lot since then and the opportunity to embed it in a small device like a Raspberry makes it really interesting for experimentation.
I am using HackRF because I had an old unit in my box... I would say it works well but the required minimum sample rate (2M) is unnecessary high (and computationally expensive) for narrow bw applications, so I would recommend anything that can work with a lower bandwidth... I read somewhere that the Pluto supports as low as 312.5KHz, so that alone would make it a better device, hardware wise.
Pluto is also better when it comes to DAC/ADC, it uses 12 bits, while HackRF uses just 8.
On the other hand the support of HackRF on GNUradio, specifically with Osmocom and Soapy plug-ins is quite good and working well, especially the TX/RX switching that works out of the box.
I have not tested the Pluto on GNURadio so I wouldn’t know about it.