The biggest factor by far in terms of conversion ratio is Your Company and Your website. There is a huge difference host to host.
The main point to consider when looking at modes of advertising is the quality of the traffic (this will have a big impact on its ability to convert) and how much it costs per prospect.
How can you evaluate the quality of traffic from a source? - evaluate where they get their traffic.
Search Engines / PPC is targeted because it is people who have strongly expressed a need to buy.
A hosting resource that gets fed by the above - further improves the targeted nature of the traffic because they have hopefully been further qualified to not only be looking for hosting, but that your service matches their needs. Find the balance between quality of prospect to price.
Then - assuming you are getting cost effective quality traffic - the biggest factor is can you convert them - but don't think of it in terms of which quality traffic converts better - this is a mistake until you have a lot of data - then you can evaluate that to further improve your ROI.
Think of it as an equation :
Sales = Quality of Traffic (eagerness to buy) X Your Ability To Close The Sale
It doesn't matter how great you can close a sale if the prospect has no interest to buy, and it doesn't matter how ready & eager the prospect is to buy if you can't close the sale.
The right process is to aim to evaluate how well you can close a sale & constantly adjust. To do that start with traffic you can be sure is of decent quality - WebHostDir etc etc and hold that part of the equation constant (or perhaps a few sources so long as you can clearly tie sales to the source - this is KEY). Once you have a grasp on your own ability to convert - fine tune by finding the source that gives the best ROI.
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