The white flowered one ('Alba') has lighter green leaves, but both have flowers that look just like those you know from begonias you grow on the windowsill in the house. The pink flowered one will spread quicker and make a bigger patch, but both will spread by seed.
They seem to be readily available and a nice addition to the fall border. Not quite for the front since they can be 2 feet tall. One note, for the first time this year the deer have nibbled on some of mine, though only once and they've not come back. The deer seem to be very hungry due to over population (we have a doe with triplets this year), but some things are just not tasty enough to finish off. I'm always on the lookout for plants they don't like - like they enjoy my 'hosta salad'