The Vertex Attribute method is the original, from GL2/D3D9 era hardware, so it's recommended if you want something that works everywhere.
Thanks! In practice, do you find the limit on vertex attrib locations to be a pain? At mimumum, it seems you'll want to pass in the world matrix, which will use four. From there, I suppose the wv and wvp matrices can be calculated in the shader, if needed, but it seems wasteful to calculate per-instance matrices on a per-vertex basis. So, suppose you pass those matrices in as vertex attributes. That would mean that 12 out of 16 vertex attrib locations are used just for transformation matrices. Positions, normals, and texcoords take up three more. That leaves only one more. If you want to do skinned animation, it seems like you'd have to fall back on calculating the additional per-instance matrices in the vertex shader.