New York vs Hamburg Steinway
Steinway builds pianos in two factories, New York and Hamburg. This explains the differences, which are smaller than the internet suggests.
Same design, two factories
Both factories build to the same Steinway design, share the same serial-number sequence, and now use much of the same material. The old differences have narrowed over the years.
The traditional differences
Hamburg pianos have usually been sold in a high-gloss black polish, New York ones in a satin black, though this has changed. Hamburg is often described as a touch brighter and more precise, New York a little warmer and more open, largely a matter of the hammers and the final voicing. In practice, individual pianos vary more than the two factories do.
Which is better
Neither. Both are excellent. In the UK, Hamburg pianos are more familiar and can hold their value a little better here, simply because that is what British buyers know. If you are buying used, judge the individual piano in front of you rather than the factory it came from.