I studied physics at the University of Cologne. If you're into Astronomy the nice thing is that it is basically just next to Bonn and the Max Planck Institute for Radioastronomy (where I did my PhD). You could also go to the University of Bonn in the first place, but from my experience things are much more relaxed in Cologne. As far as I know they now also have a lot of courses in English, at least the more advanced ones. I think Cologne is especially big in Theoretical Physics. I know the Nuclear Physics Department has its own (small) particle accelerator underneath the park outside, which is pretty cool. The spectroscopy-group is excellent as well.
Depending on where exactly you are looking for a flat it will always be expensive. Public transportation is quite good (and usually included in the University fee), so you can try to find something in the outskirts of the town or even in one of the villages close-by (I lived closer to Bonn and it took me just as long to get to the University as people who actually lived IN Cologne). Cologne also has several dormitories where you can live for very little money (but you'll probably share the kitchen and bathroom with several people), but that should be true for any city with a University.
I think you should consider in which direction you plan to go, and then check out what the Universities have to offer. One reason for me to go to Cologne was for example that I was able to choose Computer-Science instead of Chemistry as compulsory optional subject, which I couldn't in Bonn (the entire system of studies changed by now, so I have no idea what it's like nowadays). Also some close topics like Geophysics may be of interest ... so unless you are already dedicated to a specific subject, maybe try to find a place where you have enough options. Or if you know what you want, choose the best place for that.
I would care less about general opinions like "It's expensive to live there", because every city in Germany is expensive to live in compared to living in the countryside, but in every city you'll still find cheap alternatives (or use public transport and live outside of the city).
Depending on where exactly you are looking for a flat it will always be expensive. Public transportation is quite good (and usually included in the University fee), so you can try to find something in the outskirts of the town or even in one of the villages close-by (I lived closer to Bonn and it took me just as long to get to the University as people who actually lived IN Cologne). Cologne also has several dormitories where you can live for very little money (but you'll probably share the kitchen and bathroom with several people), but that should be true for any city with a University.
I think you should consider in which direction you plan to go, and then check out what the Universities have to offer. One reason for me to go to Cologne was for example that I was able to choose Computer-Science instead of Chemistry as compulsory optional subject, which I couldn't in Bonn (the entire system of studies changed by now, so I have no idea what it's like nowadays). Also some close topics like Geophysics may be of interest ... so unless you are already dedicated to a specific subject, maybe try to find a place where you have enough options. Or if you know what you want, choose the best place for that.
I would care less about general opinions like "It's expensive to live there", because every city in Germany is expensive to live in compared to living in the countryside, but in every city you'll still find cheap alternatives (or use public transport and live outside of the city).