If you have a website, you need traffic. So what’s the best way to get prospects to your site? Web marketers have long — and hotly — debated the merits of search engine optimization versus pay-per-click advertising. Which should you be using?

SEO refers to the rankings you get by having a site that search engines think is relevant and popular. If your coding is done appropriately and your site has lots of incoming links, your site will rank naturally. And for free. (You don’t pay Google, Yahoo or Bing for high rankings — you might, however, hire an expert to help.)

PPC is paid advertising. Those ads at the top or to the right of searches were paid for by companies who want to be found for those particular keyword searches. Google Adwords, the most popular PPC program, works like an auction. The highest bidder for a particular keyword wins the top position. Every time someone clicks on your ad, you incur a fee — you pay per click.

If the argument were as simple as free versus paid it would be an easy decision, but it’s not. Here are some pros and cons of each.

PPC advertising is fast, measurable and easy to control. Your ads can be running the day they’re created, you can turn PPC advertising on and off, you can control your spending by setting a budget, and you can measure what’s working.

Sponsored ads often are shown at the top of the search engine results pages, so they get high visibility. It’s not hard to set up a PPC campaign, and it’s somewhat similar to traditional marketing, so there’s a low barrier to entry.

That low barrier to entry is also one of the disadvantages — anyone with enough money can out-spend you on PPC advertising. It often is more costly over time because you have to keep running the ads. When you stop paying, your website traffic stops — the biggest disadvantage, in my opinion.

It’s often cost-prohibitive for local businesses to compete with big-budget national companies for local keywords. And while it’s not rocket science, you can waste a lot of money if you don’t know what you’re doing.

SEO builds website traffic long-term and is less costly over time because the work is cumulative. Once it’s in place, it’s going to build on itself over time. Additionally, more people click on natural search engine listings (88 percent) than paid listings (12 percent). And SEO essentially is free.

Building a site with optimized coding immediately puts you at an advantage over many of your competitors. Initial optimization and link-building might take several months, but you can often do it yourself after that, maintaining your search engine rankings for a long time. You might pay a few months for professional help, but after that, you might be able to reduce or stop spending altogether until you need a boost in rankings again.

The need to hire a search engine expert is one disadvantage; many Web designers don’t have high-level SEO expertise. And it takes longer than PPC, perhaps not much longer, to create that reputation with Google.

My own personal preference is for SEO. I cringe when I see site owners pay out month after month for PPC advertising when they could easily do well in natural search results with a few months’ worth of SEO work.

But for many businesses, it’s not an either-or proposition. A common strategy is to implement both, using PPC advertising until the site ranks well without it. Or you might use SEO to get your site ranking well for your primary keywords, and use PPC on an as-needed basis for seasonal sales, new products or services.

Local clients Doug and Ed Stansberry of Premier Paper (www.PremierPaper.com) found a brand of multipurpose carbonless paper that outperformed name brands at a much lower cost, so they decided to sell it nationally.

Keyword terms “carbonless paper” and “carbonless forms” are quite competitive, with estimated average “per-click” costs of $2.59 and $5.77, respectively, in Google Adwords. With a total estimated cost about $25 a day, or $750 a month, for PPC, being found in natural search was a more affordable long-term alternative.

So they began an SEO campaign before their new e-commerce site was even finished. Their site was done in early November, and two months later, it ranked No. 2 for “carbonless paper” and No. 1 for “carbonless forms” — obviating the need for PPC.

They’re now testing and tweaking conversion ratios to make sure their paper sells well before resuming any traffic campaigns. (Don’t forget this important step — traffic is worthless if nobody is buying.)

SEO will be their primary method for maintaining their position (and getting to No. 1 for “carbonless paper”), but they’ll use PPC for time-sensitive promotions.

If you do choose to do both, make sure your PPC person and your SEO person share information and implementation strategies. If you manage your own website, never make changes to it without telling your PPC or SEO person; you might make changes that adversely affect what they’re doing or their ability to measure progress.

Each medium has its own benefit — the trick is to allocate your marketing budget so you get the maximum benefit.