I have no experience with the 500+ (actually, no hands-on experience with -any- pi, though it's on my bucket -list to get some form of pi and give it a whirl) but I have to say that, as long ago as the Commodore 64, I thought it was a bad idea to have the computer in the keyboard. I want to be able to move the keyboard around on my desk, get it out of the way at times, maybe even kick back and have it on my lap. Even just a single USB cable hinders that and adding in video, power, network, sound, etc cables means it'll be a PITA to move it around.