Those are both nice units. Personally, the 7897 would be ruled out by the fact it plays MP3 but not WMA (most of my music is ripped to WMA). The features of the 9815 look a lot like those of my deck, the Pioneer DEH-P750MP Premier which I love.
You should be able to get the 9815 for around $350 or less online, check using froogle.com. I found my pioneer at etronics.com for $100 less than the local stereo shops were selling it for.