I would recommend you check out my recent Casio PX780 review: Casio PX780 review
As far as the Yamaha YDP181 goes, it's really quite basic (same cabinet & sound system as the V240) and has almost none of the functions of either the Yamaha DGX640 or YDPV240. But it was not built for that as it's main purpose is to give you a good replacement for an upright piano which is what it tries to do and is certainly much better in that way than the other two Yamaha pianos. However the YDP181 does not have a USB ipad/computer output connection on it which is a big drawback in my book. Even the YDPV240 and DGX640 has the USB connector although it is not the new "plug & play" system that the new Casio pianos have which is even better and makes interactive play with an iPad more intuitive. If you have not seen the latest available music & piano educational apps available for the iPad, then you should. They make learning and practicing fun and enjoyable while speeding up the learning process dramatically.
At the end of the day you can be happy with any of these digital pianos because they all do a good job. But some are just better than others in their price range and that's what you need to be aware of when you make a purchase decision. At the rate digital piano technology is going, regular acoustic upright pianos may become a thing of the past one day and it's just a matter of time. But until then, acoustic pianos are still a great investment and can be a very enjoyable playing experience although they have high upkeep costs, are heavy, loud, and cannot be played with headphones, nor can they be connected to laptop computers or iPads. No wonder digital pianos are far outselling acoustic upright pianos these days:)
For more info on digital pianos and how to get one of these models for LESS than any advertised US price on the internet, please contact me at email@example.com or call direct at 602-571-1864