Which website should you use when searching for a home to buy? If you are selling, which site should your home be posted on by your estate agent? A new website owned by the estate agency chains, OnTheMarket.com. is challenging the dominance of Rightmove and Zoopla – and it is Zoopla that is losing out most.

OnTheMarket launched in January and sparked intense controversy by forcing agents who use it to choose between the other two instead of listing on all three. It says that 90% of agents who have taken the choice have since dumped Zoopla rather than Rightmove.

Guardian Money this week tested the three sites, checking the number of properties they each list in various locations around the country, and found that the launch of OnTheMarket is already having a striking impact. For example, in Harrogate, Zoopla was this week listing only 138 properties compared with 343 on OnTheMarket, and 529 on Rightmove. But in Exeter, OnTheMarket could only muster 171 properties compared with 719 for Zoopla and 726 for Rightmove.

In some parts of the country OnTheMarket is virtually invisible, such as Bury in Greater Manchester and Dundee, where we found it listing a tiny fraction of the properties available on the other sites. But in one location, Aberystwyth, OnTheMarket had more properties than either of the other two. Zoopla points out that sites may use different definitions of a town area, which will affect the listing numbers.

Why would a seller be happy to exclude potential buyers? They will lose out as they won’t get the best possible price

Zoopla

OnTheMarket says it is confident it will elbow Zoopla aside by the end of the year, with Rightmove in its sights after that. It is also trying to make agents list properties exclusively on OnTheMarket for two days before they appear on either Rightmove or Zoopla, but with so far limited success.

Guardian Money also tested the total number of properties available on Zoopla before and after the launch of OnTheMarket. On 19 January, a week before the launch of its rival, Zoopla was listing 325,956 properties for sale in England. On 4 March it was listing 265,231, a fall of 19%.

Shares in Zoopla have come under pressure. The website came to the London stock market last June, with its shares reaching 269p in July, valuing the company at around 1bn. But this week it was trading at around 172p, a fall of a third from its peak, and substantially below its original float price.

In a statement to the stock market in mid-February it said: “The number of agency members has been impacted in the short term by the launch of Agents’ Mutual [On the Market] and its restrictive ‘only one other portal’ rule. The number of total advertising members at the end of the period [the four months to the end of January] has fallen 11% over the past year to 16,967.”

Behind the launch of OnTheMarket is the staggering profits made by the existing incumbents. Rightmove hauled in 167m from charges to estate agents during 2014, and made a 122m profit – a margin of 73%, which is almost unheard of in other businesses. So far, its dominance appears untouched by OnTheMarket. As recently as 22 February, Rightmove recorded its busiest ever day with 59m page views, while its popularity was confirmed by Google, which said that Rightmove was the most searched for UK business in 2014.

So which site should you search if you are buying, and – perhaps more importantly – which site should you be insisting your estate agent uses to obtain the best price when selling?

If you are selling

Rightmove has to be the site your property is on. In 2014, inquiries from home-hunters to estate agents on Rightmove were up 19% to 42.8m. The site sends out a million alerts every day to potential house buyers who have registered an interest in a particular area and price band, and is the only one with more than 1m properties listed in total. It says 19,304 estate agents list their properties on the site, and only 260 offices/branches have opted to leave Rightmove since the launch of OnTheMarket. If your agent is not paying to list your property on Rightmove, you should be asking why not.

Zoopla, despite the threat from OnTheMarket, still has a vastly higher number of visits from potential house buyers than its upstart rival. It says sellers are at serious risk of losing out if they go with an agent who lists on OnTheMarket. Chief executive Alex Chesterman says it is seeing 45m visits a month, compared with just 1m at OnTheMarket: “Consumers should be outraged at OnTheMarket with its restrictive and regressive practices. Why would a seller be happy to exclude potential buyers? They will lose out as they will not get the best possible price for their property. It is why three out of four agents have decided not to go with OnTheMarket, and why there is three times as much inventory on Zoopla than OnTheMarket.”

But OnTheMarket chief executive Ian Springett claims Zoopla is now on the ropes. “The figures being quoted by Zoopla with regard to traffic levels at OnTheMarket.com are simply inaccurate,” he said. “This is an attempt by Zoopla to intimidate agents who have chosen to remove all of their properties, and corresponding advertising expenditure, from them to list with OnTheMarket. We have also been told by many member agents that having removed their properties, it has become obvious just how many leads were previously duplicated, and that leaving Zoopla has had no impact on their business.

“Suggesting sellers may not get the highest price for their home because of less market exposure is a complete myth without any foundation. There is no getting away from the fact that Zoopla has suffered major losses in the numbers of agent customers they have, and the numbers of available properties they list.”

If you are buying

Rightmove remains the obvious first choice, as its coverage of the market is unquestioned, and its mobile phone app is very user-friendly. About the only risk to buyers from concentrating solely on Rightmove is that OnTheMarket gets agents to list exclusively for two days before they appear on any other site. The founder agents behind OnTheMarket were Knight Frank, Savills, Strutt & Parker, Chesterton Humberts, Douglas & Gordon and Glentree, and they have subsequently been joined by Kinleigh Folkard & Hayward. These agents tend to have a bias towards higher price and London properties, so if you are looking in that bracket it would be worth checking OnTheMarket.

Zoopla argues that buyers are not just interested in the listing alone. It points out that OnTheMarket calls itself a portal “for agents by agents” that has few of the tools that buyers want in their property search. So, for example, Zoopla will tell you when the property was first marketed and its price reduction history – which are key facts for buyers when considering how much to offer – but OnTheMarket does not. Zoopla and Rightmove also make it easier for buyers to see how close a property is to the local train station.

Finally, buyers should remember that some properties, especially in fast-moving markets, never make it on to a property portal, so it’s worth signing up with estate agents to hear about them first.