ooo.. that's a toughie. i adore MAC shadows because they're so pigmented and there's about a million colors to choose from. with Nars, their shadows are pretty but most are very sheer. for example, i love the way Ashes to Ashes looks in its container, but trying to get the color to look that way on requires applying a lot on. i usually have to put a layer of Bare Study p/p on first and then pack it on. i also think their choices are a bit limited. Nars shadows are also a bit more expensive (if i remember correctly they are $22 vs $14 for MAC) personally, i say go with MAC! you get a lot more for your money since their shadows are so pigmented so a little goes a long way. =]