Hi @Michel_Edward,
First of all, everything related to the String Library depends on your needs and budged. 
Unfortunately there isn’t a single string library that fulfills all our needs, so that’s why most of us having 2 or more libraries (not only for strings).
The Xsample Strings sound much decent than NotePerformer. Actually the strength of the N.P. is it’s AI engine, but the samples are extremely poor. I find it very useful when I’m using Dorico on laptop, or writing for live musicians.
If you would like to invest in hi-end String Libraries check the following:
- VSL - Dimension Strings Bundle (For VI Pro player) /My main String Library/.
Pros:
- Amazing and very detailed sound.
- The only library on the market that has real Divisi.
- Very rich of various articulations.
- It has patches for Solo Quintet
Cons: - Expensive
- There are no modern music effects
- There aren’t Expression Maps provided by VSL, but there such by third-party company (on sale):
https://www.articulate-preset.com/articulate-map
- Modern Scoring Strings, or L.A. Scoring Strings 3 by Audiobro
Pros:
- Great sounding libraries
- MSS has more articulations than LASS 3
- Both having only a section Divisi function MSS - Div. in 2, and LASS 3 - Div. in 3
- MSS has modern music Effects.
Cons: - The Divisi is very limited
- Both aren’t cheap
- There aren’t Dorico Expression Maps produced by Audiobro. Only for Cubase.
- Orchestral Tools - Berlin Strings /another library that I’m using/.
Pros:
- Amazingly decent and detailed sound.
- Probably the articulation richest library.
- Tons of modern music effects.
- Has First Chairs library that can be used for a Quintet
- Expression Maps for Dorico are available in this topic:
Expression Maps for Dorico 3.5+ - #30 by dspreadbury
Cons: - No Divisi possible in this library
- It’s expensive
- No Expression Maps from Orchestral Tools for Dorico
- East West Hollywood Orchestra Opus Edition
Pros:
- Has limited Divisi function
- Has many articulations.
- Has some effects
- There are Expression Maps for Dorico in the topic I’ve posted above.
Cons: - The Divisi is limited
- /I don’t have much experience with this library to share more information/.
- Cremona Quartet (solo instruments) by e-instruments.
Pros:
- Very Good choice for Quartet works
- Pretty rich of articulations
- There are Expression Maps for Dorico created by me. The could be found at the very same topic above.
Cons: - There are no modern music effects
This was just a brief overview. I hope it’s helpful!
Of course there are many other libraries on the market, so I would recommend you to spend time checking and comparing them.
Of course the Divisi function is one of the key factors if we would like to create something that sounds realistic.
Would be a game changer if NP4 provides an option to use third-party sample libraries, in a way that Staffpad does. Or if the VST team at Steinberg make such AI (plus additional manual control) player that will do it.
Best wishes,
Thurisaz