Unless you are looking at the graphics, or have them on the screen like the screensaver, I think they should be the same, but I haven't tested it.
If you're looking at the graphics it can slow down the processor, especially if its on-board video, since it has to render the graphics. Depending on the video card though, it might not slow it down much at all (I think i read the grapic is Open GL, while most video cards don't support it, but I could be wrong)
I also have a question about the command line client. Is there a way not to have it in the window? I can minimize it but it is still in the task bar...