Two brakes. If you're willing to run a rear brake, then why are you concerned about the look of two brakes? Doesn't running a rear brake "break up the simplicity" of your bike? And why would you put yourself in danger (SS with one brake is kind of dangerous) just to maintain some meaningless aesthetic? Function over form. Safety first. Etc.
I was using just a front brake on my bike when I was riding single speed, the more is way more efficient on the front (it's where the weight's transfered when braking) than the back. I reckon that if you're gonna be riding S/S then you really need both. If you're riding fixed I think just a front brake is grand, just make sure you don't grab too hard at once though!
I actually had a very dodgey SS, if I stopped pedalling, the cassete would spin off of the hub! From the odd time I'd forget, it was chewing apart the thread on the hub! That's when I moved the brake from the rear to the front and it was alot more safe..