Dynamic. You can edit one thing to affect multiple pages at once. It also lets you do more and make a better backend to work with (for updating catalog, etc.)
Plain HTML pages may have a slightly better search engine ranking, though (at least versus php pages with odd characters like ?s in them).
One other option you might look at implemeting would be SSI (Server Side Includes). These are very easy to use and can be used to replace frames or for anything else where something is repeated across several pages. It's certainly not as capable as php, but its very easy to use and definately a better alternative to frames if you were considering using them.
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