Just going to throw an idea at you, ya might wana consider geting a used professional keyboard from E-bay or a used equipment dealer. Maybe
a Korg M-1. With luck you can get one for 250. Its a professional grade snyth, just a bit dated. Used to be a very popular workstation, actually it still is. Got a built in sequencer I think.
here's one on ebay
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2507659173&category=29552
The casio and yamaha wont be as sturdy or flexiable as the M1, which was designed for studio / performance use. The M1 is a synth too, so you can make your own sounds, or use the default and edit them to taste. It should have all the sounds your looking for (or you can download them) and if you want to do some really funky snyth sounds, the m1 is a good way to go, in your budget.
The patches on the yamaha and casio will probably sound better out of the box than the M1.. but if you want that piano to sustain a little longer, or you want to make the strings sound a little fatter, you cant, your stuck with what they give you. And remember, this is not studio gear... they might be noisy and what not. I dont know, but its something to look out for.
If you need a GM able synth however the yamaha or casio would be a better choice than the m1.
Anyway, if I had to pick one of boards your looking at, I would go with the Yamaha hands down. I have never bought a yamaha product that let me down, and their pro-grade boards are f**king awesome. Other than that, the casio seems to have the same features, but there are way more sounds on the yamaha. Which one sounds better? I bet the yamaha does. And yeah, the yamaha and even the casio will be worth your 200 bucks. So would the M-1.
Hope I helped more than confused.
Gunther