On the margarine vs butter - if I'm substituting out for butter, I use cooking oil, rather than margarine, because I don't end up with the same 'wrongness' of flavour. However, I've never particularly liked margarine, so if it is something you like, then you probably won't notice the off flavour. Instead, I use one of rice bran oil, grapeseed oil, or macadamia oil, depending on what I'm cooking. For a crumble, probably rice bran oil.