Take it at an IDC. Most IDCs offer both Divemaster and Instructor IE prep training, and you can expect that things will be very standardized. In my experience, if you do DM with a private instructor, they tend to "tailor" your training to their own preferred style of diving and may press thier personal habits on to you, both good and bad. Private instructors tend to hold expectations that they’re training THEIR next assitant and nobody needs that awkwardness when you’re trying to focus on learning. I had an instructor who wasn’t great with regard to his business plan of exploiting my certification for his benefit more than mine - your mileage may vary.
You’ll also end up paying more on average for private instruction. I wish I’d have done my DM at an IDC instead of privately, because I really got hosed on costs Especially if since I was planning to be an instructor and DM, to me, was simply an intermediate objective toward that end state.
Having said that, I am a private instructor not affiliated with a dive center and I welcome the opportunity to train Divemasters. I pride myself in assuring my students that they have an entire world of opportunity in front of them to follow their dreams - not mine. :)
Diving should be fun and the minute it’s no longer fun to you: It’s time to find another hobby or profession.