Each one has it's good point and down-side. The salesman was feeding you a line. A well constructed wood arbor with care will last forever. They have vynil and plastics now that are nor affected by sunlight. Metal; if it's properly primer and painted will not need any paint for many years. With everything out there; you pay for quality.

It's all about personal preference. With vynil/plastics; it's set it; forget it. With metal; you have strength and design possibilities hard to achieve with wood. A lot of folks use only wood because to it's natural look and feel.

I have 2 wood arbor and had them for 5 to 6 years. Never had to do anything to them, but they were built with pressure treated wood.

I've had metal and wood arbors and found no trouble with either. The wood arbor is made of cedar, the metal was painted, but didn't rust out. You can get powder coated metal that will hold up very well, too.

I have made two arbors in the past four years for my wife's gardens. i made them with white cedar. The first one is left natural and will turn a soft silver eventually.
The second arbor and approximately 100 foot of fence is made of white cedar also but oil stained white.
If you follow the link that I will provide below then scroll down the page to view Cindy's Gardens you can see that wood can be a good choice.

You can see how the arbors were constructed by folling this link on my site
http://www.superwoodworks.com/ProjectsIndex.htm

Here is a link that might be useful: Cindy's Gardens

I had a Southern red cedar arbor for my Lady Banks rose, and the termites took over in 3 years. So much for the adage that termites do not bother cedar.

thank you everyone.I am even more confused by some of the pretty designs you had out there.;)