Both sets have the same functionality. The differences are
The “notation” set always has the key switches on the lowest MIDI notes, but the “playback” set has them close to the lowest note of the individual instrument, which is nicer if you want to play the instrument “live” from a MIDI keyboard with a limited number of keys.
The “playback” set always used the standard sustain pedal controller for “legato” (again, for playing live), but the “notation” set uses a different controller number to avoid clashes with playing back “real piano pedal marks” in the score.
In the “notation” set the pitch bend is consistently +/- one octave for all instruments. In the “playback” set it varies according to what the real instrument can do, to make live playing easier. Since (AFAIK) Dorico doesn’t yet play back anything that involves pitch bends, it’s not clear what implications that difference might have, if any - but presumably other sample libraries have different pitch bend ranges anyway, so there should be some way to deal with it in the Expression Maps.
If you are creating your own Expression Maps, it probably doesn’t really matter which you use, but if you also use your MIDI keyboard to enter notes in step time, it can get annoying (in any notation program, not just Dorico) if you play a note just off the bottom of the instrument’s range and activate a keyswitch when you didn’t intend to.
Since Dorico can’t yet record “live playback” of MIDI controllers, “Notation” would be the obvious choice, but assuming Dorico will be able to do live MIDI recording eventually, that makes the choice not quite so obvious in the long term. But converting a set of Expression Maps from “Notation” to “Playback” should be simple, even if it’s a bit tedious, and you only need to do it once.