I am actually using both right now and have shot various projects using both. I have a letus adapter for my HMC150 and a canon fd adapter for the GH-1. From my experience the GH-1 is pretty good in low light. With the stock lens I think the performance is almost the same as the HMC-150, but then again, I normally light all my shots/anything I shoot.
As far as mixing in post, they look pretty similar except there are some color differences, but if you have FCP, you can use the three way color corrector to get it to look pretty close. I don't think most people would notice once you get it close.
From my experience, no one can tell that 2 different cameras were used when everything is said and done.
