I've been looking at a number of clan websites lately and, having Stewarts on both my parents' sides (albeit way back), I've been checking out the Stewart sites:

The Stewart Society (of Scotland)

Clan Stewart Society in America, Inc.

The Stewart-Stuart Association of Nova Scotia

Take a look at the photo galleries on the first two sites, as well as the picture of the president of the Nova Scotian association. The most popular tartans for kilts seem to be Hunting Stewart, Old Stewart, Black Stewart, and Royal Stewart. I don't know if this affects your decision. It wouldn't affect mine, as the Stewarts on my dad's side lived in Angus, the Stewarts on my mom's side haven't been traced across the water, and I've always been fond of the Stewart of Atholl tartan anyway, even if it is associated with the infamous Sobieski Stuart brothers.

The Stewart Society has a page on Stewart clan tartans.