For birders, there's never been a better time to find a field guide. Sibley and Kauffman have both published very good guides recently, serious competition for the venerable National Geographic guide.
First, you can't go wrong with any of the three. They are all very good, although each brings different strengths and weaknesses.
Second, if you bird with a companion, carry different guides: one of you take National Geographic and one of you take Sibley or Kauffman.
Third, measure your skill level against the assumptions of the various guides. If you are a novice, then Kauffman might be your best choice. If you are a beginner who has a bit of experience, then National Geo may be your best choice. If you are an advanced beginner or better, then perhaps Sibley.
But as an overall choice, with decent art (although not quite as good as Sibley), decent identification highlights (although not quite as good as Kauffman), quite good behavior cues, excellent treatment of vagrant birds and highly readable text, National Geographic emerges as the most versatile of the three.
If you can, get all three. If you can't get all three, this is probably, by the thinnest of margins, the best choice.