Leveling up buildings should provide more impact than simply training faster.
Example
Barracks
Lv1: +5 Offense to all trainable units
Lv2: +5 Defense to all trainable units
…
Lv5: +5 HP to all trainable units
…
Lv10: +5 Dodge to all trainable units
…
Lv15: +5 Crit to all trainable units
Units that are unlocked via specific buildings can gain similar bonuses, for balancing purposes only 1 building is taken into account AND the lowest level available to train.