I'm not a Nikon guy nor do I play one on tv. ;-)
That said, I've read a number of posts in other forums concerning Nikons. Some things to consider:
- Some Nikons have great Ha response and don't need to be modified.
- Some Nikons, especially the newer ones, have better noise than Canons
- Some Nikons have something called "star eater" issue, which extinguishes small faint stars. This is usually undesirable.
Having said that, I have no idea if either of your two cameras fall into any of the above. I am a Canon guy.
I looked at the stire you pointed to, and what that shows is something called passive cooling. It will try to cool the camera to ambient temperature. It helps a little, but in the summer time, I don't believe it will be enough to make a significant difference. I could be wrong.
My take, if you're not set on just upgrading to another dslr. Keep the one you have and look for a used asi1600mc-c. That's the color, not mono camera. It has active cooling which works really well, and it's less than the price of an 850, especially if you compare both used. I have one, and having worked with modified Canon cameras before, it's a significant improvement. You will need a laptop/pc to control it, yes. That's the one drawback of all dedicated cameras, you can't just use an intervalometer.
And btw, the colors I get from the asi camera are much better than the Canons. I don't know if it's the chip used (a Panasonic, used to be on Panasonic cameras which are known for good color) or the way the raw data is created, but I love it. The yellows especially stand out, something no modified Canon was very good at.
This is just my opinion of course.
Daniel