I dunno, to me neither Scott Caan or Leo DiCaprio have a menacing-enough look for Sonny. I think Wahlberg really has that hard-ass look down and would be the best pick for the younger Sonny.

But another problem would be - what to do about the Santino Jr. part? Double-casting Wahlberg in both roles might work (with a good makeup job) but it would probably be better to cast one of the other guys as Young Sonny and then let Wahlberg play Santino Jr. in the 1980s/90s "epilogue" sequence.