I am a classical pianist who travels alot, so I had similar questions a while back. Although I try to keep abreast of what is on the market, my knowledge is limited to what I could find and try personally. Here’s my opinion:
The digital piano I use the most is a Yamaha P-88. It still seems to be the best compromise between sound, touch and portability. It has no built-in loudspeakers, no controllers and limited sounds, but as a piano it feels and sounds quite good. I don’t know if they sell it anymore, but it is almost the same as the p-140, which has built in loudspeakers.
I also have a Kawai ES4 which sounds very good, has a lighter but still firm touch and more sounds. It has built-in loudspeakers and is thus heavier.
My third piano is a master-controller, the Doepfer LMK4. It has a few controllers, which comes in handy, but I don’t like the feel of it so much. It is a Fatar keyboard, which, despite the advertisement, doesn’t seem too good to me. I use it when I have to fly because it is built for travelling.
About two months ago I tried a Casio CDP-100, which is phenomenally cheap. I was very impressed; for the price it sounds very good and feels more like a real piano than many keyboards with a much higher price.
My experience with sound modules and software samplers is that each keyboard design will enter different timbre zones at different times; the sample-zones have to be calibrated to the individual piano. My doepfer does this in reverse, it maps real velocities to corrected velocities. This could be one way to adress the problem.
Another aspect is physical stability; the heavier the piano structure is, the more it seems like playing a grand. Since my three pianos are light and don’t have the structure of a clavinova or similar, I built a custom table/rack which gives them the height and stability I want.
