It's a question I had wondered in the past, yet have never really gotten a clear answer to, not even when trying to Google the answer. Which is better, ad units or link units? Which will make you more money? Sure, there are multiple answers to this question depending on who you ask, and there are pros and cons to both, but nobody's given a clear answer yet, at least that I've noticed. There's only one way to get a clear answer to this question, and that's experience. Experience and experimentation (well, ok, so that's two ways. Here's candy, smartie.)

First, what are the pros and cons?

Here's what I've found to be the pros and cons of link units:

The pros for link units would be, they take up much less real-estate than ad units, and their use runs almost no risk of making a site or blog look "spammy". They can have a higher CTR. Often, when someone clicks a link unit and goes to the Google landing page with the search results, they might see more than one sponsored link that they like and click both or more, thus giving you a much higher CTR. Sometimes, the ads that show on the landing page can have a higher CPC than those that show on your sites, especially if your keyword targeting isn't so hot.

The downsides to link units? It takes two clicks to get you any money, rather than just one click. They also tend to be far less consistent than ad units, and seem to me to be easier to ignore. And even though you sometimes get ads worth a lot of money per click, you more often than not get ads that are worth cents to the click, rather than dollars.

What are the upsides to ad units? More consistent clicks. You're better able to control the quality of ads that show up via keyword targeting. Usually, your ads will bring in a relatively consistent amount of income compared to link units. And it only takes one click to get you any money, as opposed to two clicks. The ads are more noticeable, as well, and more customizeable (e.g. different ad unit shapes, image, text, image and text).

The downside to ad units? Well, they can look spammy, for one, or too ad-heavy. When you get too ad-heavy, you can get ad blindness. Also, putting too many of them results in a higher chance of worthless clicks (whereas the chances of dollar clicks vs cent clicks are exactly the same with link units no matter how many of them you put on the page). You can't put them in as many different places and make them look as good.

So what is best? It depends, again. It depends both on the volume of readers you get, where they find your webpage from, and what your subject matter is. On my highest-volume site, I get almost no link unit clicks - most of my clicks come on the ad units. I can go one or two weeks at a time without getting any link unit clicks.

On my lower-volume sites (I'm taking 10 viewers a day or less), I get far more clicks on my link units, but almost no clicks on my ad units. Here, the subject matter is far different - the high-volume sites are bodybuilding related, and the low volume sites relate to government jobs and trucking jobs. Most of my clicks are from links elsewhere, as opposed to clicks off of Google searches, because my Google pagerank stinks and the competition is too high to really get anywhere in any short amount of time.

So for some reason, most of my clicks on those sites, come from my link units. Maybe the link units just serve more attractive ads, or maybe my link units on my bodybuilding sites serve ads that nobody wants to click.

So what is the conclusion? Easy - use both types of units. It's pointless just to choose one or the other. Use the link units only, if you have a site where you can't give up real estate to ads. If you wanna test, go a week or two with just ad units, and then follow it up with a week or two of just link units. See what your income, CTR and CPC are. Then, use that to figure out what you'll do.