Hi - I'm usually an LV girl but I dabble in MJ, and especially love the Stam. Just for backround I myself wear a lot of black and have a fondness for it. Personally, for whatever reason, I love, love, love the Ivory Stam. Although IMO I think the black would be a better choice if I had to choose one or the other. I think it's more classic. I think it's the kind of thing where if you love both, but you buy the black, you would be thrilled to have it - period. But if you bought the Ivory, you might always wonder what it would have been like to have the black one. To my knowledge, the Nappa leather is considerabley lighter, so that my help you decide.
Picture yourself with one of them and think about how it makes you feel to not have the other one, and vice versa. Sometimes that helps me!