The CP11 is not the most modern instrument. It does, however, have some bithchin' voices. 4, to be specific. Two "piano", and two "harpsicord". The harpsicord voices are trebley, and the piano voices are fuller, with more bass. I like to crank the bass knob, hold a chord, and, with the harpsicord selected, toggle the piano buttons on and off. this makes a neat cutoff sound that is hard to describe. I used this technique on "Wash", right before the chorus.
The portasound is one I haven't played with as much. It's electric piano voice is really interesting in the lower registers. It's bassy, but most of the audible signal is a couple higher harmonics that give it a percussive sound. I used this voice on "September". I also like the flute, and it has a neat synth section. It's limited, but that keeps it from being confusing. I like an instrument that know's it's role, and doesn't fuck around.