I advertise on all of them. Just listed with Flipkey and have started getting inquiries. Vacationhomerentals is working hard to become known. My least productive site is VRBO. Advertising is far cheaper than paying realtors commissions which runs 10% in my area.
I have my home listed on all 4 sites. I think it has a lot to do with where your home is located. In 2012 here is my booking order. To be fair to Flipkey I have only had it for three months this year while the other I have had for many years. It's intresting to note every person that booked through VacationRentals I had some kind of issue with them.