At featherweight? Wladimir is P4P 50 times more skilled so I have no doubt that he would defeat Valuev with relative ease.
Tagline would be "Battle of the giants", "King Kong vs Godzilla", "Eastern monsters facing" or whatever.
Wladimirs best punch is his jab! Not his most devastating, but his most useful. He really knows how to exploit his reach advantage (which he usually has), he is a very smart and highly experienced, fast and accurate fighter. His jab is like a baseball bat, it´s really effective to keep the opponent away. Besides his left hook and straight right are two of the most dangerous weapons in the world of boxing. They can put anyone to sleep.
Valuev mainly relies on power and reach. Hard to say which of his punches is the most effective one. Maybe also his jab or his straight right.
If the fight goes the distance, I don´t think this would make the difference. Both fight for Germany and live there, and both are very popular in Russia.
Over 15 I don´t think so. But possibly Wladimir would finish Valuev between 13 and 15 because of the giants empty gas tank. Both are usually trained for 12 rounds, but I guess Wladimir - as the impressive athlete that he is - could easily go 15 as well.
Now something general: Wladimir has an awesome defense, but still a doubful chin. He´s a smooth technician, a huge boxer with outstanding skills. And he´s more powerful than his brother (Chris Byrd who faced both said so!, and he has to know it as he got hit by both)... Valuev has a good chin, but is way too slow at all to really danger Wladimir. His technical skills are poor. He can only win by a lucky shot which would be highly unlikely. I would put my money on a late stoppage or unanimous decision with Wladimir winning the fight.
Please excuse my english, I´m german