Some watches look great with a leather band. I wouldn't put a Casio digital on a leather band, but a simple, classy watch will look great. I have watches with leather bands and a couple with stainless.
Don't worry about it getting smelly. Unless you have some serious problems, a good quality band won't get smelly at all. It will start to get a patina to it and look real nice.
[edit]As a consideration, stainless looks great even when casual. Leather tends to have a more formal appearance.[/edit]