I think it partly depends on the location of your vacation rental and a number of other factors.  My rental is a condo in Panama City Beach and our busy booking season is underway.  I currently list on VRBO and just started a trial on FlipKey.  I've had 77 inquiries in the last 30 days and just a couple on FlipKey.  I used to also list on HomeAway a number of years back and found that VRBO was much stronger for me.  I may give HomeAway another try to see I can fill a few more shoulder season weeks.  Another big factor in my case is that I'm the 3rd condo listed for my building (Seychelles).  It definitely pays to try to get the highest position possible.

Jeff Bailey

http://www.vrbo.com/94029

Creator of Vacation Props, an innovative iPhone/iPad app to manage vacation properties and quickly respond to inquiries

http://www.fourthframe.com/FourthFrame_Technologies/VacationProps.html

http://itunes.apple.com/us/app/vacation-props/id452612798?mt=8

I hate to say this but I think you are going about determining the best site for your advertising dollars wrong.  Of course this is only my opinion, but this is how I decide which site is best.  I do not concern myself with the number of inquiries, especially with now that homeaway and vrbo has made it easy for a renter to sent out emails to several owners.  What I do is divide the number of bookings that I get off a site with the cost of the site to determine my cost per rental. Example, $350 a year to advertise on a site, I get 4 bookings….cost per rental is about $87……..$100 for a year and 4 bookings is $25….you see how it goes.  In theory this will tell you the real cost of advertising on all your different sites.

I did this on HA, VRBO, Flipkey, and a few other local sites. To my supprise, my local sites averaged $20 per booking. HA was $70, Flipkey was $90, and VRBO was $50, but that is not a truly fair comparison as I took a special $99 a year on VRBO to try it out and I am listed on the 3^rd page, but still got 2 rentals.

However, if you get 20 listing a year at $70 per rental, that is still better than 5 at $20. So this is just another tool to help you see the big picture of your advertising cost.

Hope it helps....Lord knows we need all the help we can get.

Kudos to you for tracking your results. Once you have plenty of data (to show a wide spectrum) I'm a big fan of evaluating your lead sources and then ditching the 80% of sources that don't produce and focusing exclusively on improving your top 20%. Here's the full 80/20 case study I demonstrate: http://www.vacationrentalmarketingblog.com/the-8020-rule/

- M