Mk3s is indeed great. I tend to swap nozzle often (different sizes, hardened vs brass) and the Nextruder makes that way easier. I also don't have a space where I can print overnight so the additional speed was extremely useful to me.

The Mk3.9 probably isn't a price competitive upgrade though, today I would in your position probably look at selling the Mk3s for 100-200 EUR and buying a Core One instead. But if the Mk3s works for what you are doing, then don't bother. (The Mk3.5 might make some sense if you only want a bit higher speed and network connectivity.)

Personally I don't see myself replacing the Mk3.9s for a long time, though an enclosed printer would be nice and let me print warp-prone materials like PC and ASA.