@barbiedolphin:
Kubrick "just" installs / restores stock / factory Kindle OS (actually stock firmware that is also jailbreak-ed).
If you've decided to stop using Duokan then that is was you should do: start fresh, gain jailbreak, customize stock firmware.
Depending on what features of Duokan you were using thus far stock firmware + added
KoReader is a valid alternative to those "I want to be able to read epubs on a Kindle" & "I want a better PDF reader software" types of scenarios.
Note the difference in approach:
1) install custom OS ( Duokan)
vs
2) add an app to stock OS. (Koreader is just an app, that you start using custom launcher named
KUAL)
You can judge yourself what fits your use-case better.