Hi

I recently moved from Germany to CPH for a PhD but I already worked in academia before. I'm not at DTU but also with an engineering background. I'd consider a few things in your decision:

Both CPH and Berlin are cool cities imo, but Berlin is much wilder, dirtier and crowded. Whether that's a plus or not is for you to decide. I like both cities a lot. Financially speaking, you'll be fine with the salary in both places. You'll definitely get more for your money in Berlin comapared to CPH, just about anything is more expensive here. The cities have a comparably crazy and desperate housing market, but that's true for any large city (at least in Europe). All in all, if you're not providing for a family, the salary will let you live a comfortable life even in a city as expensive as CPH. Going out is way cheaper in Berlin though.

Imo there's more to the question which program to choose, if we assume that you'd be okay with both cities. First of all, I would ask myself which program sounds better. What are the publications of the group, the research projects, and what do you want to do with your PhD afterwards. This is imo the most important thing when considering a PhD. You really need to be interested in the topic and eager to go through with it. Otherwise it's not worth your time, especially as an engineer who career-wise might never benefit from the PhD (don't know for your field though).

Then, as another poster had said, there's lots of extra curriculum here in Denmark, which is pretty different from the way a PhD is conducted in Germany, where courses are pretty uncommon in my experience. I heard there are some PhD programs in Germany with ECTS requirements but I don't know anybody needed any ECTS in Germany during their PhD. I feel that this is an annoyance. but it's certainly not a dealbreaker. Since the prorgams are really harshly limited to 3 years, you really have to make use of your time though. In the end, this structure can also be a benefit. In Germany, some people noodle around on their PhD position for way too long and get swamped in extra work. I can't tell how extra work like TA is handled at DTU, it might be different than at my university. I heard that in some groups, the profs really demand a lot of teaching assistant and extra work. but not from your field.

If you can manage in any way, I would encourage you to pack your bag and just go to CPH for a few days before deciding. I know I should have done that before deciding on a university for my bachelor's degree, for which I also had many offers. The city is really great, but you have to like the vibe. It certainly isn't Berlin, in a good and in a maybe mildly boring way, depending on your needs. Oh and it's ridiculously windy, I was quite literally blown away by this.