I lived in Berlin for 3.5 years, I was born in Manchester, UK where I lived for 20 years and I now live and work in London for 3.5 years.
Developed has many meanings but reading through the other answers i’ve picked a few and rated my experience in each. I’ll put the winner in each case in bold.
Infrastructure
Germany has a more developed infrastructure system such as rail, trams and buses. It runs to almost all small, medium and large towns, these systems work well together and are on time / run all night. Only London rivals Germany in this regard with London tube being on par with Berlin on cost, interconnectivity and ease of use. Small towns often have a single railway station in England but the trains and the connections will not be as good as London or the whole of germany.
Social Development
Equal in my opinion. When I was in both countries and growing up I felt that I had equal opportunities to engage with other people, to learn, to meet other people, to feel safe, to be on a level and to be encourages.
Safety
Germany felt safer as growing up in Manchester it could be pretty rough. Manchester has some quite bad poverty and this led to crime. However, I disagree with people that say London feels unsafe. i’ve always felt super safe, even my girlfriend does 40 minute commutes back every evening on her own and on public transport with no issues. I never feel unsafe in either country. American on the other hand….
Costs
Equal. Even in London I think you can get a lot done on very little. I find that Transport in London for the distances you make is cheap, but I do find drinking much more expensive in the UK. Good places to eat out are better and often better value (with a discount voucher) in London, discounting was not a big thing in Germany. Germany wins on most basic necessities, but I found rent in Berlin compared with London to be very equivalent, the only difference is the space you would get. London would be around 50% smaller for a similar price, but there are just not as many large apartments compared to Berlin.
Healthcare
Equal. was very similar in both countries. In the UK healthcare is 100% free, you don’t pay directly it’s taken out in your paycheck and does not scale with your pay. In Germany it is proportional to your pay and increases as you earn more. You also have some small costs such as 10 euros to visit a Dr but after that it’s free. German Doctors surgeries are less busy however as NHS is overloaded. The moral of the story here would be for the NHS to charge a bit more / proportional taxation as people earn more so that they can invest and bring down the overall wait times. The quality of treatment was almost exactly the same, I had good Dr in both places.
Culture
UK. I might be biased as I grew up here, but especially London has a huge amount going on that uncomparable to almost any other city on the planet. If you can’t find it in London it doesn’t exist. Berlin has a huge culture, but I found that it repeats. Everything ist so Berlin ya?! Whereas I find the UK more eclectic with, more odd or different viewpoints. Again this is just my opinion and it very much depends on what you’re looking for.
I loved all my time in all the places i’ve ever lived and really I don’t think there is a right answer to the question. You know what you like and what you’re used to. Both Germany and the UK are amazing places to live and work and you will get out exactly what you put in! Try them both!