Kent's Phytoplex contains a preservative (EDTA most likely) to keep the product from going bad. As long as the bottle isn't stored in the back of a '78 Pinto in Las Vegas in the summer I don't think you need to worry about shelf life before you finish the bottle.
As for the cost difference, Phytoplex is cheaper, and easier to ship, and store, however LIVE Phytoplankton is IMO always preferred and more nutritious. Phytoplex sells for about $1.03 an ounce, while DT's sells for about $1.33 an ounce.
So the cost difference is not that huge, few bucks on average. Though you will pay more usually to ship DT's since it must go Priority Mail or UPS Next or Second Day Air.