Plants can be pruned either toward the end of fall or the beginning of winter. Prune back about 1 to 2 feet to prevent flower heads from drooping too much.
You are mixing two completely different hydrangeas,H. paniculata(HP)and H.arborescens(Annabelle).
The common is that both bloom on NEW wood.habit,sun requirements,size,form of bloom and pruning technics are different.