Nearly the same piano on paper
The Casio CDP-S110 and the Yamaha P-145 are the two default answers to "I want a real 88-key digital piano and I do not want to spend much". They list within a few percent of each other, and a surprising amount of the spec sheet is identical: 88 keys, ten voices each, 64-note polyphony each, a damper pedal in the box and no stand.
Where they separate is the key action, the physical size and the loudspeakers. The size gap is the clear one, the speaker gap is narrow, and the key action is the thing you touch every day.
A note on method: this is a spec-based head-to-head. We have not played the two side by side, so instead of ruling on feel we set out what each maker built.
The action, and why it is not settled by a number
The CDP-S110 uses Casio's Scaled Hammer Action II, finished with a simulated ebony and ivory key surface. The P-145 uses Yamaha's Graded Hammer Compact, usually shortened to GHC, with a matte finish.
On our spec-derived touch scale the Casio grades one step above the Yamaha. That number comes from published construction, not from playing them, and one step is not a gulf — but the textured keytop is a real, checkable difference at this price, where most keys are plain plastic.
Set against that, GHC is Yamaha's genuine graded action: heavier in the bass, lighter in the treble, in a shortened case. Players who intend to move to an acoustic piano or a Yamaha console later sometimes prefer to stay inside one maker's touch. That is a legitimate reason to choose the P-145 and no spec sheet will argue you out of it.
Size: the Casio is the slimmer instrument
Both are around 132 cm wide and both weigh between 10.5 and 11.1 kg, so on width and weight they are interchangeable. Depth and height are where the Casio pulls ahead.
The CDP-S110 is 232 mm deep and 99 mm tall. The P-145 is 268 mm deep and 129 mm tall. The Casio is about 3.6 cm shallower and 3 cm lower. That is a smaller gap than the Casio's slim reputation suggests, but on a narrow desk, on a shelf, or slid under a bed or a sofa between practice sessions, a few centimetres in each direction can still decide whether it fits. Measure the space first: this is one of the rare cases where the tape measure, not the spec sheet, gives the verdict.
Neither runs on batteries, so "portable" here means light enough to carry, not free of a power socket.
Speakers and what you get to play
The CDP-S110 puts 16 watts through two speakers; the P-145 puts 14 watts through two. That is close enough that neither will sound obviously louder than the other in a living room, and neither holds much reserve before the tone starts to harden. Speaker output is not the tie-breaker here.
The Yamaha answers with built-in repertoire: 21 pieces against the Casio's 10. The Casio answers back with layer and split, which the P-145 does not offer — so on the Casio you can stack piano and strings, or divide the keyboard for teaching, and on the Yamaha you cannot.
Both give you ten voices and 64-note polyphony. Sixty-four is the honest limit of this price class: it is enough for most solo repertoire, but heavy sustain-pedal writing with thick chords can clip the oldest notes. If that describes what you play, the answer is to spend more, not to choose between these two.
What neither of them does
This matters more than any single spec, because buyers routinely assume otherwise.
Neither has Bluetooth — not Bluetooth Audio, not Bluetooth MIDI. Both connect by USB MIDI and both work with their maker's app over that cable, but you cannot pair either one wirelessly to a phone to play along with a track. Neither has a line output, neither records internally, and neither has a lesson function.
Each has a single headphone jack without headphone optimisation, but they are not the same socket: the Casio's is a 3.5 mm mini jack, the Yamaha's a 6.3 mm standard jack. Full-size headphones normally end in a 6.3 mm plug, so they go straight into the P-145 while the CDP-S110 needs an adapter; with earbuds or small on-ear headphones the situation reverses. For quiet practice either is adequate rather than good; a console at three times the price will be noticeably more comfortable over a long session. And while both include a damper pedal, neither includes a stand, so budget for one.
Who should buy which
Buy the Casio CDP-S110 if the space you have is genuinely tight. It is about 3.6 cm shallower and 3 cm lower, it adds layer and split, and it carries the textured keytop and the slightly higher touch grade. On paper it is the stronger instrument of the two, though by a narrower margin than its slim profile suggests, and not on speaker output.
Buy the Yamaha P-145 if you want Yamaha's Graded Hammer Compact action specifically — because you are heading toward a Yamaha acoustic or console later, or because you have played GHC and liked it — or if the larger set of built-in pieces suits how you practise. If you already own full-size headphones, its 6.3 mm jack takes them without an adapter.
For most first-time buyers the Casio is still the marginally easier recommendation, but the two are closer than they look. The case for the Yamaha is the key action, and that is exactly the argument you should go and settle with your own hands rather than take from a table of numbers.