It’s completely pre-determined, as are most improvements of both troops and heroes.
The only place where you get to choose a path, is with heroes learning new talents.
And the only random of such improvements, is the chance for heroes’ special skills increasing when leveling up – and even then, it either remains unchanged or improves in a pre-determined manner.
ETA: I found a table of these pre-determined skills per level for 4* troops: Kerb's 4* Troop leveling chart, hurray!
I don’t know if similar exists for 3*, 2*, nor 1* troops.