First of all, and I know the Kurzweil piano/string sound, you will really like the KF90 piano/string sound. It is a really great piano/string. You have the ability to program in the characteristics you like in a piano. Roland gives you paramaters such as cabinet resonance, string resonance as well as other parameters to make the piano "behave" more like a real piano than i've seen on other pianos. I am very familiar with the SK880 as well, and I don't think the sounds in the SK come close to the KF90's. Also, the Roland is much easier to navigate. I've sold both of these keyboards and again it's my opinion the KF90's piano action is far superior, the cosmetics, the built in speakers, which the SK doesn't have and the great combination of features in the KF90, makes this keyboard number 1 in my opinion.
If you can live without a hard disk drive, I think this would be a great choice for your next keyboard.
The KF has a great on board vocal harmonizer, and can load and find midifiles and styles while others are playing. There is a very good 16 track on board sequencer. The touchscreen works very well, although some have objected to the number of screens sometimes needed to change things. Once you've become familiar I don't think this will create a problem.
Any more questions please let me know.