No. Depending on the quality of the judge, they could accept that the company have treated you reasonably, but they've got to change their advertising in the future to stop any misunderstanding, or they could back you and tell them to give you the better disk - it's too much of a risk. I agree that they haven't played fair, but you have to consider the battles that are worth wining.
At the end of it, I don't think you'll notice that difference in speed, but as I've said earlier, how will the company act when the computer goes wrong, so I'd still suggest you see if you can return it, and you're close enough to Christmas to wait for the January sales.
On a dafter note; I follow astrology, and now is not a good time to enter into contracts or buy electronic equipment - wait till January 1st onwards.