I have the Nokton and I used the 50/2.5 for about a year, they're both awesome. Optically you can scrutinize lenses to hell but that's not what makes a picture anyways. I find with a bit of processing I can intermix with the Nokton from the same project and no one will know (but I mostly stop down a bit and never really shoot color).
So option one, just stick with your Nokton, save money. Or at least look at the real reasons why you're not happy with it. I see you're a small lenses kind of person (good), so read on
Go for the Summarit if you want: super small and light, no finder blockage, smooth focusing ring (unlike the Nokton), comes with tab (doesn't need to bake sculpey to make an ugly one for the Nokton), short throw (quick autofocus), never needed a hood for it. Nice and contrasty, fairly smooth. Not an effect lens, gets out of the way, just delivers. I don't think you will carry both the Nokton and the Summarit at the same time.
Price: look at the forum, there are a few for a bit more than a grand. Buy the old one, you lose less money if you decide to sell, no one wants it as everyone is dreaming about lux and cron.