If you are dealing with only one late-paying company, hire a collection agency that's in the debtor company's area. The local agency may have dealt with the debtor company before and know the company's finances and the best way to collect from them.
If you have past-due accounts from customers in multiple locations, hire a national collection agency, or one that is located in the same city as your company to ease the reporting process. It's rarely smart to deal with more than one collection agency at once -- you'll just end up confusing yourself and mix up which company is trying to collect which debt.