also strategic creatures should have some sort of upkeep
They have, see below:
The system is basically the same as in Master of Magic. I think it is perfectly reasonable as it is, maybe some balancing needed in the general mana numbers here and there, but the system itself is fine.
First of all, you have a difference in cost for both types of summons, since you have to pay a mana upkeep each turn to keep the creature on the strategic map. Apart from that, summoning a creature in tactical combat is a big advantage and therefore should cost the same amount of mana minus the upkeep, because you can go over the max unit limit in a stack which you cannot on the strategic map.