I played both at Guitar Center (stores all over USA) and spend about 30 min each and CASIO sounds better all around. Especially the pianos and ep's are probably better in CASIO. Speaker is no contest - it's better with CASIO. I think the Yamaha model is a 61 keys IIRC.

I thought the Yammie model looks more like a toy. I own Yamaha PSR2000 which costs around $1200 when it came out.

Just go to GC and print your ads they will match it and you can try both keyboard for 30 days. There is no better way to know a keyboard than spending one week with it and then decide.