Two types of collection agencies

There are two main types of collection agencies, those that operate ONLY on a percentage of what they collect and those that charge a flat rate per account placed or sell the right to place a certain number of accounts within a certain amount of time for a flat fee.
I'm a member of several credit and collection organizations and groups and this morning a PR piece about an agency was part of one of those groups. That agency charges $5-$15 per account placed regardless of dollar amount and they claim that a gentle nudge is all most people need to pay and that's why they can charge so little.
As a 20+ year veteran of the collection industry, I have learned that a very, very small percentage will respond to letters from a collection agency. Phone calls are what generates payments, if letters and invoices did, there would not be over 5,000 collection agencies in the US alone.
As a contingency agency, J P & C gets paid ONLY if we collect for our clients while the other type agency gets paid whether they collect any money for you or not. As a collection agency owner, I know that they cannot afford to spend any money on phone calls, more that a letter or two or trying to locate a missing person that owes the money and stay in business very long.
While it may seem like a bargain to pay $15 for someone to collect a $1000 account, the bottom line, who is more motivated to do a good job for you, someone who already has your money beforehand or someone who relys on getting results for their clients BEFORE the agency can get paid?