For compact Arduino functionality in "permanent" projects I prefer the RBBB (unless it's something like my weather station project, where I needed shield compatibility to make it easy to add Ethernet). It's small, cheap, and doesn't add the cost of the USB chip to projects that don't need it.
Note that you'll also need something like their P4 board or BUB to connect the RBBB to a PC for programming. But I only need one to program the 5 RBBBs I have, so it's a big savings if you're planning to make multiple gadgets.