Hello.
No, they are not the same.MotBW is based on a campaign that you slowly conquer shard,meet and chat with other masters.
Of course the main plate of the game is the same more or less.There are differences like the increased combat map size and no similar campaign in Imperium but it is based on the same formula.
They are based on the same engine so.....crashes,errors and bugs are more or less the same.

It's like the MotBW sandbox campaign had been separated in 2 modes:
_ A purer story campaign where you choose few
_ A pure sandbox with quite no story
So MotBW may be still prefered if you want to play the richness of both modes at same time.

Eador IV: MotBW on a new (documented) engine with the added features of Imperium: 5 stars, no less.