As I understand it and practice it,
1st aspect:
If you don't use Nickel (original Kobo software) anymore and you prefer KOReader or Coolreader from vlasovsoft or Plato and you like the new and wonderful UltimateMangaReader -> KSM.
If you still prefer Nickel and you sometimes need KOReader to read a pdf -> KFMon.
2nd aspect:
If you read new books -> Nickel.
If you read old books from Gutemberg Project, pdf, rss feeds or mangas -> KOReader.
I don't buy books. I don't steal them too. Everything I'm interested in is old free books from public domain. Now I'm reading Alexandre Dum' ass (Shawshank redemption) Dumas, why would I want to pay for this? Sometimes I want to read news articles or mangas, so my choice is KSM, KOReader, etc. The only reason I still use Nickel with my good old 3.19.5761 patched firmware is for Wikipedia offline with Kiwix.
3rd aspect:
If you want to hack your eReader completely, add ssh/sftp, create your own apps -> KSM (and all the rest as already said).
If you don't want to hack everything, you're afraid to hack too much your Kobo -> KFMon.
4th aspect:
If your Kobo is a very recent one, H2O2 v2, Clara HD, you don't have choice, for now -> KFMon.
There's something for everyone.
And thank you NiLuJe, especially for your Python package, with tshering you are my two hacker masters.
