Hi Joerg,
There is no difference yet. But now Cisco has different BUs for these two models and it is believed that in the future 760x series is going to be carrier class equipment with more L3, telco and so on features, while 690x will be developed as enterprise class switch. The first step to this is the IOS 12.2 SRA release, which are supposed to work on 760x platforms only( eventhough you can still run SRA on 690x :)) .
Off course any Cisco guy could tell us more on this.
cheers,
Blagoy