DIY for sure, although I see benefits to both! My interests when traveling personally tend to lean towards stuff that's not usually covered by tour groups, and I'm lowkey a control freak when it comes to itinerary-building.
However, I did do a school group trip with a pre-planned itinerary when I went to China, and because we went to three [huge, often overwhelming] cities in a little over a week, I felt it was the best way to get my first introduction to the country. We managed to hit all the touristy/historical bits and they definitely kept me on my feet! I definitely plan on visiting again with a DIY itinerary though, so I can see what I missed!
I'm also personally a fan of the whole 'free walking tour on the first day' bit, but not sure if that counts.