On the surface of it I would keep the Astra and get the money for the BMW.
However.... how is the water level in the engine on the Astra and are you getting any smells on the interior? The misting could be a leaking heater matrix. Not too bigger problem if you are prepared to keep topping the engine up (and dealing with the steaming) but it may need repairing. If that's the case and you aren't prepared to do it yourself then it will make keeping the car uneconomical. It could also just be a leak (a seal or gromet that has expired), which would be easier and cheaper to repair.
If your ultimate goal is a Porsche however, bin both and take a foray in to the world of bangernomics. There are tonnes of very good cars out there being sold for next to nothing, that would fulfill your needs, and you could run for a couple of years for no money at all. I know, I've done it.
Best of luck.