Assume that your demo will be viewed / listened to on a stereo device, unless they've actually ASKED for your demo. Vimeo does not support 5.1, though, so people that want it can hear it there. (see below)
Also assume that no one is going to ASK for your demo (someone will, don't worry, but assume that no one will). They're going to go to your website and maybe watch the first 30 seconds of it, and make their decision from there. They're not going to download something to burn it to a DVD and play it in their home theater -- if it's not streaming, they're not going to give you even 30 seconds.
So, in my opinion, if you plan on developing a 5.1 demo, make it your only online demo, and make it available somewhere where surround streaming is supported. Make it also compatible with stereo systems. Don't give someone the option of "Surround Sound Demo available on request," just make that the only option.
UPDATE: Official word from Vimeo is that they don't currently support surround sound, and they don't comment on upcoming features. BUT, Flash 11 just dropped, and that does support up to 7.1. (Also, Microsoft Silverlight supports surround) I would expect Vimeo to support surround sound in the near, if not very near, future.