Can anyone tell me the difference (if any) between buying RSN Hage-Finnentrop Route for TSW2 and TSW2020? And if there are no differences, why is there a better discount for RSN as tsw2020dlc?
No difference. The TSW 2020 one is cheaper because it is the original and was released years before the TSW 2 one, so stream made it cheaper.
The main difference is that the TSW2020 version is out of support, the TSW2 version is in support and will recieve updates. If you own TSW2020 (you can't buy it any more) - buy the TSW2020 version of RSN, and you'll get the TSW2 version for free. I thought they'd sorted out the discrepencies between TSW2020 and TSW2 DLCs.
The TSW version of the route is a straight port of the original TSW2020 version. However, as people have said, the TSW2 version gets updates, whereas the 2020 version is no longer supported. Also, as people have pointed out, owning the older version means you get the newer one free.