This year's Super Bowl pits two longtime adversaries in their most ferocious competition ever and we don't mean the guys with helmets on their heads. During ABC's Jan. 27 broadcast of Super Bowl XXV, Coca-Cola and Pepsi-Cola will launch remarkably similar ''interactive'' ad campaigns and give away big bucks in the process. In Pepsi's ads Ray Charles will invite viewers to dial a toll-free number for random chances to win up to $1 million. Coke's spots (announced just weeks before Pepsi's) will star Leslie Nielsen: Viewers can win up to $1 million by holding up decoder game pieces (obtained by purchasing Diet Coke) to the TV screen during a special halftime commercial and deciphering a hidden message. Both ads will be hugely expensive ABC is reportedly charging $800,000 per 30-second spot. Who's likely to win this latest round in the cola wars? Maybe no one. ''The effect of the campaigns is diluted when both companies use the same tactics,'' explains Clive Chajet, chairman of Lippincott & Margulies, a New York image consultant firm. ''These ads may just cancel each other out.'' Now that would really be a flat ending.