You can decide the mesh depending upon your problem statement. If your problem is simple go for triangular. If you are solving complex case then triangular mesh would be failed over there. It has nothing to do with the accuracy. It's the problem you are solving. For example it is impossible to solve a supersonic flow with mach no >1 with triangular mesh.

-Faizan

Quad map is much better for the simulation but often you need to partition the domain in order to create such grid. Tri or tethrahedrons allow to automatically also complex geometries but the quality of the numerical solution can suffer a little.

Franco

If you consider the accuracy result only, the accuracy of solutions in quadrilateral cell shape is higher than triangle cell shape for the same cell amount. Because of the triangle cell shape has just 3 vertices and 3 sides but the quadrilateral cell shape has 4 vertices and a basic 4 sides. However, the calculating time, cell amount, 2D or 3D model, and complex geometries should be considered to decide an appropriate model for simulation.

Aphaiwong 

Thanks you very much Mr @Muhammad Faizan-Ur-Rab  @Franco Concli  @Aphaiwong Junchangpood @Abhishek kumar Pathak 

Really helpful from all of that answer..thanks again..