There a lot of boards that outperform BBB 10-20-30x times. But both RPi and BBB are more than enough to run an autopilot code.
One of the great advantages of RPi over BBB is 1080p camera module with hardware H264 encoder as well as bigger community.
Well, SSHing may be not that useful for those who just want to fly a drone, but it's quite useful for developers, researchers and enthusiasts. Compared platforms are way less powerful and won't be able to perform new advanced flight algorithms, as well as they do not provide easy network connectivity for 1080p H264 video stream from Raspberry Pi's camera module.
Each approach has it's advantages. Navio was built to serve as a Linux platform for APM which provides more processing power than microcontroller platforms - it's useful for advanced flight algorithms (for example, a lot of new features of APM asuch as EKF are not supported on older APM hardware) and the level of integration between payload apps can be much higher than on two modules with serial link.
Navio does NOT weight 24g. It's weight is around ~12g.
RPi A+ with Navio = 23+10 = ~35g.
Pixhawk weights 38g.
With Navio+RPi you get a lot of stuff compared to hardware like APMMini: network connectivity (LTE, long-range WiFi), affordable 1080p camera module with H.264 encoder for FPV, a lot of processing power for advanced flight algorithms (as an example - a lot of new features of APM such as EKF are NOT available on old APM platforms) and the overall hackability, which you don't get with any microcontroller based autopilot.
Thanks! We've been thinking of it, but rejected the idea. RPi A+ is quite light, so RPi+Navio weighs as the normal autopilot. Also, A+ is much cheaper.