They are broadly similar keyboards, but the 9 series has more features. The 7 series is a kind of 'cut down' 9 series, but still excellent keyboard. However, I should point out that the true comparison would be S975 to S775. Gets a little harder when you compare an older model, S970 to a new model S775. mY guess is that the store is getting rid of the 970 at a decent price because the S975. A detailed review of the features would be needed to see if any of the missing features matter enough to her to pay out the extra. Its hard for us to say what matters and doesn't matter to someone else, but both are good keyboards. But I would also look at the S975, because that us the latest and greatest in the range.
Mike