I think Chad's example and solution to Fine Control and Improved Defense work well. The reason for the restriction on command cards is that +1 Attack skill is more powerful than +1 dice in my opinion.
The math is pretty straight forward. A (+1) Attack die bumps the die roll 25% for damage possibility, +1 Attack skill bumps it an additional 16%; on the average of rolling for (5) 3 3 for combat resolution.
Then of course there is the same correlation for all the range in die rolls.
So +1 Attack skill is more powerful.