They are all pretty interchangeable for most styles, though there are differences between them. I typically use Thomas Fawcett and Barids, the latter for styles like porter, stout, brown ale, and IPA where you'll cover up much of the malt character but still want that extra bit of complexity. Fawcett, Simpsons, and Crisp are all excellent MO's, though I think the TF has a bit richer flavor. I've not used Glen Eagle yet. The only one I don't like is Muntons, as it is the blandest of the ones I've used.