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 :D), but I feel there is more to it. I would not discard randomness too quickly.

I like that morale system. Basing it on reserves is pretty cool, and gives some incentive towards not keeping the field flooded as much as possible at all times.

Whether or not that randomness has any real impact depends upon, again, the pace of your battles and how important each unit is. Without knowing more, I can only recommend playtesting it - it's a simple element to remove, at any rate.

Also, I'll mention that when I include randomness into things like damage calculation I tend to go with a normal distrubution - that is to say, damage is centered around a particular mean (the unit's base damage value) and rarely deviate by much.

randomness is a fact of life in combat, and should be modeled to the degree necessary for whatever level of realism you're aiming for in your combat simulation. as mentioned elsewhere in this thread, the frequency distribution of the random values generated must be sufficiently realistic as well. so if you're shooting for a high degree of realism you should model things like random damage, friendly fire, tripping, morale, weapon breakage, etc.

in my caveman sim/rpg, chance to hit, damage done, chance to flee (morale), opponent hit points, and weapon wear and tear and breakage all have some randomness to them.