I can give you a hint: the level of units depends on the faction. They gain a minor combat bonus from higher levels, but typically, each faction's different-tier units start acting in different ways, either in macro or in combat capabilities. They also gain typically more slots for attachments on higher tiers.
The highest can be an okay strategy; typically, you have 2-3 options to choose from. There is usually one with the most raw power, and some with utility value (such as allowing/preventing retreat, destroying enemy attachments, etc.). Going with the highest power is not wrong.
Do you mean attack in the strategic map or in the tactical map? In combat, there is no advantage to the attacker. The defender has the advantage that the attacker must select a combat tactic first, and the defender can see what the attacker chose. But no numeric difference. Some factions may have a tactic that is effective against enemy bases, a rare attacker advantage.
Thanks for the insights!