I think which one depends on your intended use. I'm not a keyboard specialist, but I had to get one when I went back
to music school at age 57 in order to be able to practice at home (with headphones).
For my use, I did get the $500 casio, it was quite satisfactory. If I were going to put it to heavy use for a long time
I might have wanted to get something a little more rugged. I did some some expired listings for the entry level yamaha
on E-bay for $200, but nothing came available during the time I was looking. Both the casio and yamaha had midi-outputs, so
if had been really pic fy about the way it sounded, I could have bought a sample set for use with a computer.
If you're intending to play for other people , then you'll need to decide whether to lug around an external speaker with you
or something that sounds good enough with the built in speakers. And also whether you intend it for use only in your
home, or if you're going to travel with it.
One of the good things about brick-and-mortar old-fashioned music stores, or even guitar center (which usually does
have a keyboard department) is you can try out instruments right there without any obligation to buy.
(I'm an oboe player, and I'll just mention that the entire oboe section of the philadelphia orchestra was playing on yamaha
instruments for many years, and skywalker sound (part of the lucasfilm complex) up in marin has a big yamaha (Non-digital) piano as one of their on-site instruments).