bruge or dinant?

8 nights in either place is too long. Bruges is good for a couple days at most, so there’s no reason you couldn’t stay in both places. If you’ll have a rental car, there are more places you could visit as day trips from Dinant.

I love. Brugge could spend 3-4 days there then daytrips and bike rides would be great.
Think Ghent etc.

Dinant is great for a day trip but I do not know about spending a night I would split it up Brugges Ghent or Antwerp

A particular reason for choosing Bruges (Brugge) is that it's nearer three other major tourism cities of Belgium: Gent, Antwerp, and Brussels. (Not to mention Leuven and Lier.) The low-cost, unreserved local trains between them are fast and easy to buy at the last minute. Any one of those four cities has "more to do" than Dinant. Dinant is certainly worth visiting, but it is farther away, and really far from Bruges, which is perhaps the most popular destination (if not my own favorite, which is Antwerp.) No one (except maybe a saxophone player (invented there) will ask you if you went to Dinant. Everyone will ask you if you went to Bruges.

If your major interests are those four prime tourism cities, you never need to change hotels because they are so easy to get between. Do pay attention to how far your hotel is from the train station, however. The same tram/bus tickets work in Bruges, Gent, and Antwerp, while another kind is needed for local transport in Brussels.

Because Gent is a real university town, restaurants and maybe cheap hotels will be cheaper than in popular Bruges. (No personal experience.)

Both.....and be sure to visit the nearby breweries when you are in Dinant.
I scored major points with my wife (a musician) when we arrived there.
There are good places to eat and drink. It's picturesque. It's worth a visit.

I would suggest Brugges...very interesting, can't say anything about Dinant, except for its involvement in both wars, have not been to Dinant.