Hi phil ,
thanks for reaching out.
On Bela you can connect up to 8 (or 64 using the multiplexer capelet) analog (e.g.: pots) controls to Bela, and up to 16 digital ones (e.g.: buttons), I2C, SPI, Serial, Ethernet and USB devices.
As far as I can understand, connectivity on the Mod-duo is limited to audio I/O and the two on-board pots and USB MIDI. Not sure how/if they support all the standard Linux drivers through their interface.
The characteristics of the Mod-duo are a bit hidden on the website, but from what I understand they run some mainline Linux on a cortex A7 dual core.
The dual core A7 has higher raw CPU power than the single core A8 we are using, but without knowing more about the system, it is difficult to understand what latency guarantees they can provide. Bela provides reliable low-latency performance thanks to the Xenomai real-time kernel extensions.
With regard to software support, from what I see you can probably use their product without ever having to write a single line of code, using pre-built lv2 plugins and patching them together to create pedalboards, and that is great.
On Bela you would probably have to write some code, though we support (among others):
PureData, which is a graphical programming language which is pretty easy to get started with.
FAUST, which provides an online editor to connect existing sound processing modules together
Additionally, we do not (yet!!!) provide an effect library comparable to that of mod-duo, nor an easy way to plug modules together. At the moment there is no support for lv2 plugins, but it would not be difficult to implement, so we may look into it later down the line.
Last, prices are very different and reflect the target audience. The 499$ for the Mod-duo give musicians a finite product ready to go, at the expense of lack of flexibility and - if I understand correctly - somehow closed. The ~100£ for Bela give you a development platform where you can run a choice of programming languages, wide-range connectivity and a growing Open Source community.