the drivers side manifold is the easiest to access. and you don't need to read temps from boths sides. someone here maybe running dual pyrometers. but for regular usage, 1 is fine.
reading temps before the turbo actually gives you more insurance that you don't fry your PSD or kill the turbo. the temps are hotter at the manifold versus after. so the critical temp will be read sooner at the manifold, closer to the source of heat. someone here can probably explain this in more detail.