For development work, the BBB is a good choice. Here's why I would choose this (well, why I have chosen this) over the RPi...
(Some of this assumes you might want to grow into your own product. If not, it may not apply.)
- It's popular enough that it has a community, and a few distros ready-to-go. A baseline is very nice so you aren't forced to start at zero.
- The hardware design is open. You can take the design files, modify them, and produce a product without any concern. TI / BB endorse this path.
- The software is less encumbered by binary blobs. IIRC, you still need one for OpenGL (I think), but 2D is doable with OSS. This may or may not concern you.
- The CPU is available in single quantities. No need for NDAs. No need to stack SMD packages on each other. You get the parts manifest for free, and can duplicate it or examine the datasheets as required for your purposes.
- It's faster. Not by an enormous margin, and certainly not compared to a PC or late mobile phone, but it helps. (Compiling X on Gentoo was an exercise in patience. Recommend cross-compiling what you can.)
- In general, it seems to be a more solid device. Ethernet and USB are implemented fairly well. You get on-board flash and SD-based storage to choose from.
- Cost, after including the req'd SD card for a Pi, is about the same. OTOH, you will need a mini-HDMI cable if you don't already have one lying around.
If you're interested in developing multimedia (read: video) projects, go with the Pi. For audio only, the BBB might be better. The McASP is a beast with serious potential. It is nearly wasted on the BBB, but you have access to the registers and a few of the required pins, so you can at least get started before spinning your own reference board.