Essentially both the old YDP640 & YDPV240 are the same as far as controls, sounds, and electronics with one being portable and the other in a furniture style cabinet. The YDP181 (which should also be replaced fairly soon based on my best guess because it's been out for awhile) is a much more basic piano although it does have a better key action and piano sound than the other two including the newer versions. Yamaha makes reliable and fun to play digital & acoustic pianos and for many years I have played them professionally and enjoyed them very much. They are generally very well built and have a big following among musicians and families. The Yamaha company started in Japan in 1887 founded by Mr. Yamaha, and built their first Japanese piano in 1900 (over 100 years ago). They were a piano company long before they started producing motorcycles, wave runners, ATV's, snowmobiles, tennis rackets, golf clubs, speakers, stereo's, etc.
I do like the larger, easy to see back-lit screen (left pic) on the Yamaha YDP-V240 (also on the DGX660), the music minus one play-along features, the "wait til you play" lesson function, on-board music notation in the screen (although it's still small and not very easy to read), and lots of "easy-play" functions and automatic chords which many people do not use. Also, the V240 has extra instrument sounds which is cool, but how many instrument sounds can you possibly use:)? However, the the Yamaha V240 is internet priced at $1000 more than the very competitive Casio PX780 which is a current Casio model so that price difference alone makes the Casio a much better buy in my opinion. I would recommend you check out my Casio PX780 review: Casio PX780 review
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 firstname.lastname@example.org or call direct at 602-571-1864