Dry yeast and liquid yeast both have their advantages and disadvantages. Selecting yeast depends on your brewing plans and needs. It used to be that dry yeast was inferior to liquid yeast, and some older brewing books still report this information. This is absolutely not the case anymore. Dry yeast is sterile, strain-pure, and highly capable of producing great beer. Because it is dried, the shelf life is often a year or more and it is much more tolerant of warm storage or shipping conditions than liquid yeast. Dry yeast is also packaged with nutrient reserves and is ready to directly pitch without a yeast starter. For high gravity fermentations, more than one pack of dry yeast should be used. Yeast starters are not optimal for dry yeast because they can use up the nutrient reserves of the yeast. The downside to dry yeast is that not all strains can survive the production process, so there are far fewer yeast strains available for beer brewing (there is an excellent selection of dry strains for wine and mead making, however).
The range of available strains is the greatest benefit of liquid yeast. Any strain can be collected and cultured for use by homebrewers. However, because liquid yeast is a live culture, it is usually more expensive and is much more perishable. Yeast shipped by mail order usually has a practical shelf life of 3 months (sometimes longer) and can be adversely affected or destroyed by temperatures above 90 F. There are also fewer cells per pack than dry yeast, so when making a beer with a gravity above 1.060 (or when making any lager) the yeast should ideally be “grown” by making a yeast starter before brewing day. Using multiple packs of liquid yeast can also accomplish this same goal.