According to the Wyeast web site, 3056 is a blend of a ale and wheat strains, whereas 3068 is a single strain. I expect that 3056 will produce fewer esters and phenols than 3068, so you'll get less in the way of bananas and cloves. I'd probably prefer 3068 for a hefe, but for a dunkel, you'll probably get really good results from 3056.