I would imagine the MM6 is better than the PSR 550 - I have the PSR 420 - which is basically a general midi instrument - good drums - and replaceable ROM.
The MM6 is based on the MOTIF series you can check out the models and compare them on Youtube before buying - the MOTIF is an awesome...
If you have already made the HTML - add it as an attachment to the mail - they will be able to see it in IE7+ - or upload it to a website and tell them the URL of the site - either way they will be able to see it.