I prefer brass. When cleaned and buffed with 0000 steel wool, I love the classic shine. I have had some problems with nickel cases turning slightly black when soaking them in Birchwood Casey case cleaner. Couldn't buff it out, I have to live with them. I soaked them a little longer than the instructions said, but I have done the same with brass with no problem. Probably my own fault as the cleaner is most likely meant for brass, but it makes the brass look great, even when soaked extra long.
I cannot tell any difference in case life between the two, but my brass cleans up better than the nickel, and since the brass up better for me, I don't use the nickel too much.