As a designer I would say, fixed. It's a very elegant solution.
As a designer that wants to sell his games I say go for random range.
As a player that wants to have fun I definitely say go for random range.
Someone above mentioned Diplomacy and Risk. Yes, Diplomacy is an awesome game, no doubt. But compare how popular the Risk is and how popular Diplomacy is. Sure, you can say that brainless casuals play Risk (which is actually partially true ), but I feel there is more to it. I would not discard randomness too quickly.