Boarduino eh?
pretty cool if you really use that breadboard.
Personally, i would grab a Duemilanove just because its the latest and greatest normal arduino that uses shields. Very well put together.
Plus, you can get a prototyping shield with adds a button and two leds and you can even get a mini breadboard to stick on there too.
Check out the 5th picture.
But, if you like the boarduino, id go with the USB version, purely for programming and prototyping convenience.
You know while its easy to run a plug pack in, USB is so, so easy. Its nice.
Plus, im sure there's no limitation of you being able to power it independently. i think the jumper next to the USB port is for that, but regardless, there will always be a way to power it. You just might need a regulator. still cheaper than a serial converter.
Just my two cents 