Personally, i feel like having more fun to have a gameplay restriction rather than detailing restriction. I've been able to do MC contests (and some 72h jam), and had fun with them. I attempted at doing some detailing contests, but it was really bad, including the 72h jam (my 1st entry).
Subjectively, a gameplay restriction would make the contest open to more people. There is definitely more people able to make decent gameplay than a decent detailing. Sure, there would be more questionable quality maps, but there would be a lot more good maps! Also, i think in TF2 despite the look of most of the recent maps that are stunning, i think the gameplay still is the main concept of what makes a map good. Making a detailing contest with gameplay in it would make a lot of maps mostly unfun to play on despite their good look.
So i'm casting my vote on gameplay restriction and rehashed contest prompt if only it's mainly about gameplay.
However, i do like the idea that was done for the TFC Contest of having two categories, Gameplay and Detailing. I feel like it could still make most people happy since there would be some being better in gameplay or some better in detailing. And at the end, the main goal could be to get both, so it could still let the maps go towards a good gameplay and good look!