A digital piano is going to have more piano-like keys, especially if it's a decent digital piano that has "graded scale weighting" on the keys, which means the lower keys feel "heavier" and the higher keys feel "lighter" in their response as far as how forcefully you have to strike them, as on a real piano.
In contrast, most keyboards have either unweighted synth- or organ-style keys, sometimes called "diving board style" because they're flat with perhaps a little plastic coming down on their sides and fronts. Some keyboards have box-shaped keys which look piano-like, but they are usually only semi-weighted, so even though they might look more like piano keys it doesn't really feel like you're playing a piano.
Another difference related to the keys is that a digital piano typically has 88 keys like an acoustic piano does, although some models might have only 76 or 73 keys for people who don't have room for a wider 88-key digital piano.
In contrast, most keyboards have only 61 keys, although some models might have 76 keys for people who need a bit more than 61 keys.
As far as sounds, many digital pianos have only a dozen sounds, or perhaps up to two or three dozen, which typically include a few acoustic piano sounds, a few electric piano sounds, a few organ sounds, possibly some other keyboard sounds like harpsichord, and possibly some acoustic guitar, bass guitar, and string section sounds. Even though there aren't usually that many sounds, they are generally high quality.
In contrast, most keyboards have at least a few hundred sounds, and some models may have close to a thousand sounds. These typically include everything you'd find on a digital piano-- although the quality might perhaps be lesser than on a digital piano-- as well as electric guitars, brass (trumpets and other horns), reed instruments, saxophones, flutes and other woodwinds, a large variety of synth sounds, and a variety of drum sets.
Another difference is that digital pianos are often designed to be able to take three different pedals as on a real piano, whereas most keyboards are designed to take a sustain pedal and possibly also an expression pedal.
These are just some generalizations, and they aren't always true-- particularly with regard to the number of sounds-- so you just have to check the specifications for any particular models that you're interested in to see what their specifications are.
As far as price, most digital pianos are going to cost a good bit more than a keyboard, partly because of the number of keys (88 versus 76 or 61), but also because of the type of keybed (graded scale weighting versus semi-weighted or unweighted).
I think the only digital pianos you're likely to find that are around the price of a PSR-EW300 will probably be something like the Williams Legato III or the Williams Allegro III. I'm not sure whether they're supposed to have graded scale weighting or just semi-weighting, and I have no experience with them, but I've seen comments from "serious" or professional keyboardists and pianists who basically don't think Williams digital pianos are very good, either in sound quality or in the responsiveness and feel of the keys.
If you go up a bit more on your budget, you could consider the Casio CDP-135 or CDP-S100. However, I think those models are said to be not quite as good as the Casio Privia models, with the most affordable model being the Casio PX-160.
However, all of those Williams and Casio models have 88 keys.