I have the 300 disc Sony. Works great and has many playback features that would be fun if I had time to program it.
You can also link the Sonys together, ie have the 300 disc CD and the 400 disc DVD players running off a single remote and working together for quick playback during random listening.
I've run out of storage in my 300 disc, so that linking feature will probably come in handy.
Oh yeah and they are VERY deep so measure the space where you're going to put it before buying.