Honestly, the nations of this setting are not developed enough to be able to give a satisfactory answer to this question.
Here is a brief list of the information we'd need to know before we could even begin drawing conclusions.
- How large is the entire army of either state
- How well equipped is that army
- How well trained is the army as well as the elite units
- How fast can either army move
- How does armor work in this setting
- How populous are either state
- Where is that population distributed
- How much resources can each state call upon
- How long does it take to mobilize these resources
- What are the available means of resupply
- How developed are their food storage systems
- What network of allies can be called upon
- How secure is the leadership
- What is the life expectancy of the current administration
- How likely are internal struggles going to mess up the state when the current administration dies
Now, in terms of army vs army. Demacia does have a fantastic advantage when it comes to war, since they have flying bird mounts. However, everything Riot's written about these raptor knights kinda shows that they're being used in the most inefficient way possible. So, how dramatic the difference they can make is somewhat negated.
Then we can get into the details of leadership. Napoleon may have been the greatest general of his era. And the army of France may have had more experienced men, and a far greater understanding of war than Russia. But because they didn't have the foresight on what materials to bring on their conquest they were eventually beaten back (though this is a gross oversimplification).
It's entirely possible that a war can be changed based on the decisions of a handful of people that cannot be predicted. If one side has a Yi Sun-Sin and the other side doesn't, then the impossible can happen.