While the Sigma is a much cheaper alternative you can't read it in the dark - unless you add some kind of light on your own. What Sigma ought to do is come up with a 12v lighted unit since they are popular add-ons for the m/c community.
AFAIK, the Yellow Box will correct your speedo at the expense of making the odo off, according to the speedohealer site, their product is able to correct the speedo and leave the odo alone.