I think, business/portfolio sites will always have a better chance of more sales.
That’s true for the short term, but a theme in an obscure category will have exposure for a longer period of time. Look at the only church theme on TF: http://themeforest.net/item/wordpress-church/40190. It has been around since April, and there is still interest in it as of a few hours ago, judging by the comments page. I think this theme will be the only one in this category for a long time to come.
It’s just like with SEO …there is a short head and a long tail. You can make yet another business/portfolio theme and enjoy a few weeks in the limelight before being replaced by 20 other themes, or you can be a big fish in a small pond with the smaller categories like churches and restaurants, or srhnbr’s (http://themeforest.net/user/srhnbr) ecommerce themes. I think the latter offers better long term sales prospects.