unwrapping before or after?

Um Yes, I am making a video game model and I was wondering if I should mark seams and unwrap on my high poly version or my low poly version(which will be the high poly version with a decimate modifier).

Assuming you’ll actually be applying the decimate modifier, I think it’s generally better to unwrap & map textures/materials to the low-poly version, and is essential for normal mapping from your high-poly sculpt. The low-poly has to have a UV map to receive the baked normals. I don’t think decimate will do good things to a UV unwrap, either, and any cleanup you do on the model will need be done in the UVs as well. Better just to unwrap once all your low-poly modeling is done. IMO.