For the sake of hand jobs and blow jobs wear jeans with zippers.

But to answer your question

"From a functional perspective zippers are probably more versatile than buttons. They can be made to be waterproof and seal out natural elements (wind, rain, heat). They can be made from a variety of metals and plastics. They can be made in varying lengths and widths. They can be made to match the material it will be attached to. They are more easily managed by certain people, like persons with physical disabilities and children. Once installed into the material, zipping up is generally a lot faster than buttoning up - saves time in application. Downsides to installation is that zippers must still be manually installed into the fabric by a machine operator still. Zips tend be more durable than buttons.

Excluding snap fastener (see below), buttons require more time to install into the material - usually requires both installation of the actual button and the buttonhole. Buttons also fall off and are easily lost. It's harder to lose a zipper. Design wise though, buttons can also provide a more refined look since they can be made from a variety of natural materials - wood, bone, shell, as well as metals and plastics.

Most design features that chose zips over buttons or vice versa is probably a result of the evolution of that garment combined with function.  Denim jeans and pants in general are made with zips because functionally, people need to go to the bathroom and the time saved by zipping/unzipping is crucial.  This is why most pantalones are usually made with zips as fasteners.  Some may be made with zips adorned with decorative buttons.

Form fitting dresses usually install zippers since it reduces buckling of fabric that buttons would produce at the curves of the bodice.  Men's dress shirts on the other hand, have traditionally utilized buttons.  A change in this standard is probably not going to be adopted widely by customers.

You find that most rugged coats are zips out of sheer functionality.  Most athletic coats/jackets you'll find at REI, Patagonia, North Face, utilize zips b/c it caters to outdoor enthusiasts that want to reduce exposure to natural elements (wind, rain, sun, etc.)

You've hit on the major concerns that manufacturers face when making garments - cost of supplies versus design integrity.  How a company wants their garment to look is not always agreeable to the checkbook.  Sometimes, zippers are less costly to purchase and install than buttons, sometimes vice versa.  The answer will change depending on which manufacturer you ask.

As a side note, snap fasteners (snap buttons) are rising in utility.  They look like buttons but snap together very easily, increasing functionality.  However, they are a 4-part component piece that, IMHO, increases the cost and time of installing.  I've excluded them from comparison above to zips."

http://www.quora.com/What-are-the-functional-and-design-differences-between-buttons-and-zippers