* Nike sponsors 6 of top 10 most marketable soccer stars * Adidas fighting to maintain lead in soccer market * Nike top star is Ronaldo, Adidas bets on Messi * Even top German players defect to Nike * Puma persuades Balotelli to wear mismatched boots (Adds link to graphic, analyst comment) By Emma Thomasson BERLIN, May 28 (Reuters) - U.S. sportswear group Nike is banking on its sponsorship of more of the world's best-known soccer stars than Adidas in its battle to overtake the German firm as the sport's top-selling brand at its World Cup this summer. Nike has signed six of the 10 most marketable footballers in the world, to just three for Adidas and one for smaller German brand Puma, according to a new ranking by sports marketing research group Repucom published on Wednesday. More...