The fixed is more immersive, the scale is somewhat better for gameplay, but seems to break with the towering units when you back up.
I really enjoy scale, and if you are going with the look of the game as being a seemless world, I think keeping everything relatively the same size is important. For example, having a HUGE unit should be HUGE proportionally to buildings and world features in addition to units. With the scaling, it will only be large in comparison to other units, otherwise it will pop out of the world and seem detached as though it is an icon, rather than a part of the world. It would only be when you have zoomed in enough to the point that the scale is 'correct' would the game world feel immersive again.
I think you should have fixed scale, and to help spot your units as you zoom out and they become too small, to add a banner or some other type of immersive 'icon' pop up out of their heads to make them easier to spot on the whole. But I really really think unit scale should be proportional to the features on the world map in addition to one another.
This sort of brings up the fact how I saw in screenshots the player's character standing by buildings being about 2x as high as village huts and towering over everything. It made it feel like a gameboard and really sucked you out of the immersion you are attempting to create with the world.