Choosing a WordPress theme should always be done by making sure that the theme has the features that you are looking for as well as function the way that will be best for you, your site and it's visitors.
Don't go strictly by how "Pretty" a theme looks since most themes can be made to look good with either some styling modifications or some design thought put into setting up the site.
Go for a theme that most closely fits your needs (all of them), those of the website and more importantly, the needs of the visitors that will be using your site. If you need to do any kind of modifications to make a theme fit your needs then those are usually done quite easily.
Also, don't buy a theme without looking at the publisher/developer/etc. and their update frequency, especially on theme forest! Many publishers there will have a theme added to the market, maybe update it once or twice, then abandon the theme and updating it. You don't want to buy a theme only to find out that it isn't getting updated with the latest security fixes or compatibilities with WordPress and the major plugins.
Good Luck.