First, tread carefully with Business.com as I know of companies that pulled out of there after Panda hit them and reports that they had seen a drop in terms of rank. Basically, if you are going for generic directories, try and find ones in your niche.
As for which service to use, I don't actually use PR services myself so can't offer any real input here, but do hear good things about PRWeb.
One thing to also remember, if you are looking to build backlinks, you need to diversify, but not to the point where the site has no meaning to your industry. For example, you could quite happily look for blogs and sites that sell fitness equipment, fitness food, bikes, climbing equipment and even diet websites - it is all related at some level and Google will know this.
You might be surprised just what you can turn up.
Andy