When it comes down to it, as far as high-temperature searing (as for steaks) goes, the pans are equally efficient. A skillet offers advantages for sautéing, and a sauté pan offers advantages for shallow-frying, moderate-temperature searing (as for chicken pieces), or braising. In an ideal world, you'd have both, but if I had to pick one, I'd go with the skillet, as sautéing is a step in nearly every recipe I make.