To be honest that is quite a tough one. Most of the good places tend to be a bit device/platform centric.
As to how to decide what to use then I'd say it really comes down to how much of an OS your project needs, all the platforms you mention would be capable (at various degrees of overkill) of doing the two projects you mention.
On the Pi/BB you have the advantage of doing it all in userland at a very high level if you want, and using pretty much any language.
Arduino you get simplicity and the fun of doing it relatively close to the metal, but you are pretty much stuck with Arduino's Pseudo C++ like language (though if this sort of development tickles your fancy and you want to expand to other projects you can of course move (and even port your existing projects to) a grown-up AVR development environment quite easily)
In fact for your colleagues project there is something on arduino.cc which is already 80% of the way there.
http://arduino.cc/en/Tutorial/Datalogger.
Personally I wouldn't consider a Pi in any application where I didn't need it's Gfx/hdmi capabilities but that's because outside of those I don't believe it is a very good platform :D