I always say that owning almost any kind of a keyboard or digital piano is better than having none at all because getting involved in music is the goal, regardless of how much or little you can afford. I usually don't talk much about the lowest priced portable digital pianos out there because they are generally not very good. But the Casio CDP120 (the newer replacement for the CDP100) is, in my opinion, quite nice for the money and considerably better than the Williams Allegro in almost every way. However, there may still be a few Casio CDP100's for sale out there, so this blog review still focuses on that model along with the Williams Allegro. Go here for my full review of the Casio CDP120: Casio CDP120 Review
Two weighted key action 88-key portable digital pianos under $400 are the Williams Allegro ($249 internet price - pic on left) & the Casio CDP100 (approx $349 internet price-pic lower left). There are other 88-key lower priced pianos out there that have completely unweighted or semi unweighted key actions (like a keyboard), and I would try to stay away from those if possible as far as playing piano goes. The Williams & Casio pianos I am reviewing here are "good for their price" because the prices are low. But for anyone to suggest that they are good enough for an intermediate piano student or semi-pro or pro musician, is not being honest (I have seen where self proclaimed 'knowledgeable people' have actually said this). Also for someone to say "the Allegro is also great for more experienced players due to the high quality features it comes with is just making things up in my opinion. People have also said that The Williams Allegro model "provides you with the realistic touch and feel of a grand piano in an inexpensive and portable way" is equally absurd, and I am actually quoting someone here who reviews pianos. If you really believe you can get a portable digital piano for $249 which does all that, then I'd like to sell you a piece of desert land that "I promise you has all the running water you will ever need even though their are no wells or water supplies nearby!" You cannot get something for nothing (as the saying goes), and if you think you can and believe those claims about the Allegro, then this is certainly a free country and feel free to by a PSO (as I call it) and have fun. A PSO stands for a "piano shaped object" and that's about as much in common as the Allegro has with a real piano in my opinion.
Both pianos have weighted keys with the Allegro being much lighter weight and the Casio being a bit heavier weight (somewhat better). The overall piano sound itself is good on the Allegro so that's OK, but what really separates the two is the way the keytouch volume/response is and also the way the sustain pedal responds. The Williams Allegro keytouch piano response is not smooth at all and it should be good right out of the box. The action is somewhat choppy with noticeable jumps in volume as you play the keys harder or softer. Also, the keys themselves go down much too easily almost like a cheaper keyboard as compared to a piano. An actual piano touch response is smooth and gradual, with subtle changes in volume and is heavier weighted... not so on the Allegro. This would not be good for anyone above a beginner level and even with beginners this could cause bad playing habits because you'd always have to be compensating or adjusting for it and piano teachers (like myself) don't like that. The Casio does have a noticeably smoother, more gradual keytouch volume response than the Williams, but it could be a bit better as well (keytouch is much better in the newer CDP120).
There are some other differences between the two keyboards as far as total note polyphony (64 on the Williams which is good, and 32 on the Casio CDP100 (and 48-note polyphony on the newer CDP120 which is acceptable), number of sounds (8 instrument tones on the Williams and 5 on the Casio) & features, etc, but the main things that need to focused upon are the things that I have mentioned. On the surface it would look like the Williams offers more, but actually it offers less than the Casio in the areas that are important. So it's what you cannot see that is what you really need. There is one huge noticeable difference between the two instruments, and that is how much they weigh. You would think that something this small and portable would be lightweight which is the case with the Casio weighing in at only 26 pounds...that's great. But the Williams weighs a whopping 64 pounds which is crazy heavy for a low priced portable piano! That's more weight than many portable 88-key "pro pianos" with many more features, better keyboards, better touch, better everything. Perhaps that won't be an issue for you, but if it ever falls off the stand or you have to move it, then it does become a big issue.
Learning to play the piano is one of the best things you can do for your kids and yourself. If you can afford it, do it with a higher quality instrument, but if you cannot afford it, then I recommend the Casio CDP100/120, which in the very beginning will be fine, but if you progress in your playing which most students do, you will need to move up to something that better emulates an acoustic piano (more actual piano tonal and key-touch characteristics along with better pedal response), and your piano teacher will like you better for it too:)