PaulRB:
Hi and welcome. Yes, you can power your breadboard circuit from the Arduino. But USB can only supply max 500mA, of which an Arduino like UNO itself will use less than 50mA. Thats still plenty for most experiments. Just don't go mad with dozens of leds!I would recommend Arduino over RPi for simple experiments and learning, every time. I own both and don't use the RPi for much at all.
I would also reccomend a breadboard compatible Arduino like a Nano 3, for you, as a "breadboarder" like me, rather than a shield type of person. Why get an Uno and then have to have a spaghetti-like tangle of wires connecting it to your breadboard?
Paul
I agree, for breadboarding it's hard to discount the ease of use of the arduino Nano type board. Plus it's not hard to burn the Uno bootloader into a Nano and get the same higher baudrate on upload and a small increase in flash sketch space.