A connected mesh vs a separated mesh, which one is better? (both are 1 object)
Mesh 1 - A single mesh with topology
More triangles
Mesh 2 - A plane + cylinder (not connected)
Less triangles
Better in what sense?
There’s no general rule for this, it depends on what you’re using the meshes for. Typically, less geometry is better so the second option (not connected) would be your default. However there’s situations where you’d choose the first one, for instance if using per-vertex lighting instead of per-fragment, or if you could look at the mesh from underneath and the plane and cylinder were z-fighting.
Also, how you plan to uv-unwrap and texture your meshes can also affect your topology choices. The first mesh is easy to unwrap if you want continuity between the “floor” and the cylinder as they form a single manifold surface, where the second one would be easier to unwrap as two separate uv islands.