I think it depends on how prominent you want the keys to be in the mix, and if you are playing melody lines versus primarily backing parts. It's very easy for keys to override the guitars in the mix, so if you are playing melodies on keys, I'd suggest playing single note "riffs" similar to what a lead guitar part would normally do, ala old Children of Bodom. If you want the keys to act as the background (Dimmu Borgir, for instance) then you have more room to work with chords, multiple synth voices and ambient-type effects. Of course, you can always go for a more dissonant style if it suits the song, or play harmonies above/below the guitar, taking care not to wash out the bass guitar in the process. I personally don't like it when the keyboard and guitar parts are playing the same thing, so decide which instrument will play lead and which will act in more of a backing role. In my opinion, using a choir or string voice on a keyboard is better suited toward backing parts, as it tends to sound fake when trying to play the "busy" style most people expect the lead guitar to handle.
If you are using MIDI and have a lot of different synth voices available, you'll be able to get some very realistic choral and vocal sounds.
