"Best of luck!" and "Good luck to you!" are very similar in form. The one that encourages your friend more would depend on how formal you wish to be or the amount of "luck" you wish to give, in essence. "Good luck" is a less formal and simpler way to give someone 'good wishes' and well being; whereas, "Best of luck!" is more intense and should give a stronger connection of wishing your friend the highest amount of luck you can give him or her. What it really comes down to is the sincerity. Similar to saying "I love you" to someone as an example. The difference is being whether you write it with more expression or not like, "I love you!" or just, "I love you." There's "Good". There's "Better". Then there's "Best!".