What to Consider: When you're applying polish, a base coat serves as an adhesive between the color and the nail. A topcoat provides shine and prevents chipping but does not have that adhesive effect. If you want something that does it all, look for a multipurpose clear coat that is labeled for use as both a base and a topcoat.
Bottom Line: If you simply want shiny, natural-looking nails, use a topcoat and lose the base coat. If you're applying colored polish, use each for its intended purpose.