Personally, I would still lean toward the PSR-E403, on the basis of its greater number of sounds, and the fact that you can modify the parameters of each preset sound. I'm not familiar with the WK-1200, but it has far fewer sounds, and judging from the manual you can modify sounds in synthesizer mode, but I think that applies to only a certain number of basic waveform sounds as opposed to all of the preset sounds. And while I've never heard a WK-1200, I used to have a Casio CTK-710 and it didn't sound nearly as nice as my YPT-400, so I expect that the PSR-E403 sounds a lot better than the WK-1200.
What the WK-1200 does have is one more octave of keys than the PSR-E403-- they can both play the same range of notes (you can shift octaves as needed), but having that extra octave of keys might make all the difference if you're planning to primarily play piano compositions. Also, I thought the piano sound on my CTK-710 was very good, one of its better sounds. So the WK-1200 might be the better choice to use as a "digital piano."
On the other hand, most portable organs have only 61 keys, and many synthesizers traditionally have fewer than five octaves of keys, so the PSR-E403 would probably be the better choice if you're looking for a more versatile keyboard for playing a wide variety of sounds.
So it really depends on how you plan to use it-- as a piano, or as an organ/synth-- and whether you want to connect it to any other MIDI gear without going through a computer.