They're similar. Polyphony would be a bit harder on the continuum because you've got to tune the intervals by ear and feel, whereas I believe the Seaboard quantizes to quarter tones. So, you can play chords easier on the Seaboard, but in theory the chords could sound better on the Continuum if you're really good at hitting those just-tuned intervals.

Another instrument that's similar is the Linnstrument from Roger Linn. It's very playable and last I looked was a lot cheaper than the Seaboard or the Continuum. You can turn quantization on or off. The firmware source code is also open source.