Every converter advertises accuracy, but accuracy is not one number. Monophonic sources — a sung melody, a single guitar line — convert nearly perfectly everywhere. Polyphonic mixes are where engines diverge, and where your choice of tool matters.
Cloud converters
Upload-and-wait services are convenient and often free at small sizes. The tradeoffs: your audio leaves your device, queues form at peak hours, and output rarely exposes the detection thresholds that would let you fix a crowded transcription yourself.
Desktop suites
Full notation programs include audio-to-score features with deep editing afterwards. They cost real money, install gigabytes, and reward you for learning a whole application — worth it if notation is your daily work, heavy otherwise.
In-browser models
Open transcription models now run directly in the browser tab. Files never upload, thresholds are exposed as sliders, and results land on an editable piano roll immediately. The constraint is your device: a four-minute song takes about as long as a video call on a modern laptop.
What to actually compare
Run the same 30-second clip through each candidate and count what you'd have to fix: wrong pitches, missing notes, phantom octaves. Then compare privacy (does the file leave the machine?), tunability (can you re-run with different sensitivity?), and output (MIDI only, or MusicXML and MP3 renders too?). A converter that scores well on all four is rare — and usually free.