It wont actually answer your question directly, but if you have a loook over at another ACE forum, there was a thread recently which compared all the different options available, firstly in a general overview, and then a breakdown of odds. From his results, SP was consistently the worst option, although best of SP or best tote was generally at the better end, and he thought Betfair was the best across all odds range, despite not having actual figures presented

The thread is called Best of tote actual results and can be found in the horse racing section.

Could help with your question.