Hi Rich, I don't have all the answers but I might have some (with varying degrees of accuracy)
I was actually looking at the Pisound recently and, yes, it looks interesting. I believe it is indeed BBB's PRU's that allow Bela such low latency and, as the Pi is PRUless, Pisound probably doesn't achieve the same performance latency-wise. Looking around the forums I found references to latency figure around 2-3ms (from memory)
Another potential disadvantage of Pisound is the lack of digital and analogue control/sensor inputs. For me that is a big deal.
Also, the OS options don't look as flexible/developed as Bela. I like to work in C++ and the Bela environment is perfect for me in that regard. If you like to work with PD, don't need GPIO, need the extra power of the Pi but aren't bothered about <2ms latency then maybe the Pisound is for you but I feel it doesn't have the flexibility of Bela (or the community)
There is another Pi audio project that claims latency as low as 1ms and has digital/analogue GPIO, Elk audio Pi hat with Elk audio OS. The problem with that is they have no boards available for sale and, even though I've checked a few times over the last 2 years, have never seen the boards available! They have recently added support for the HiFiBerry in their OS however. That gives you similar hardware to the Pisound but with a Juce fork giving you a nice C++ environment with a good, established community. Still, no GPIO so no sensors, pots etc (you could use MIDI but I don't dig the resolution!)
If your motivation is the extra processing power of the Pi compared to BBB then you could wait a few weeks(?) for Bela to be ready for the Beaglebone AI which will offer performance in the same ballpark as Pi4 (not sure which will be faster?). Personally, that's my plan at the mo (though I must admit I'm also asking Elk when they will next have boards available, I might do both systems!)