They overlap a bit.
But not completly: Imperator does more variety but with smaller period of time.
For instance, he did Savoy, Naples, Papal States, Venetia, while Sandris is doing only "Italy".
But while Sandris makes unit for Bronze Age (early / late), Iron age (early, lage), Dark ages, Middle ages (early, high late), 1550, 1650, 1700, 1750, 1790, 1812...
Imperator focused on 16th, 17th, 18th.
In fact, Sandris is very kind to try to match the needs of my own mod when he does new pack (although he always goes even beyond).
Although I like Imperator's units, I'd rather use only the units from Sandris to have more consistency. The only units of Imperator I planned to use are the artillery.
But this is because I'm making an epic mod. For someone who would like to focus on Europe, Imperator will bring additional units, such as Scotland for instance, or more than one German states.
I see them more as complementing each other than competiting.