Line a baking sheet with waxed paper and grease the paper; set aside. Wash and thoroughly dry apples. Insert a Popsicle stick into each; place on prepared pan. Chill. In a large heavy saucepan, combine the sugar, corn syrup and water; bring to a boil over medium heat, stirring occasionally. Stir in baking soda. Stir in butter until melted; gradually add cream, stirring constantly. Cook and stir until a candy thermometer reads 242° (firm-ball stage). Remove from heat and cool to 200°. Place 1/4 cup pistachios in a shallow dish. Dip apples into caramel mixture until completely coated, then dip the bottom of each in pistachios. Return to baking sheet; chill. In a small microwave-safe bowl, microwave white chocolate at 50-percent power for 1-2 minutes or until melted; stir until smooth. Transfer to a small heavy-duty resealable plastic bag; cut a small hole in a corner of bag. Drizzle over apples. Repeat with semisweet chocolate. Sprinkle tops with remaining pistachios if desired. Chill until set.