Spawn will beat Ares for these reasons:
1) He has fought godlike beings before.
2) 3rd Age Spawn is still comparable to Thanos and Darkseid.
3) Mammon is definately Ares's superior, and Spawn has humiliated Mammon before.
4) Even if Ares is a physical powerhouse, there are so many ways in terms of magic, greenworld's gift, and necroplasmic power that Spawn can defeat Ares.
5) Ares' one advantage is that he is the DC Earth's virtual embodyment of war. However, does his power reach spiritual level? It seems his powers are all active only on a physical level. Spawn is way beyond the physical.
6) Spawn killed Malebolgia who is definately above Ares in power.
7) Spawn survived an encounter with Kali who is comparable to Galactus and Pheonix (read my post on Kali vs Galactus thread)
8) Spawn still lived even after the Disciple ripped him to shreads, the Disciple so far seems much greater than Ares in power and authority.
9) Spawn survived a brutal torture session in Hell by Thamuz the master of all torment.
10) Ares was said to get his ass handed to him by Darkseid, and Spawn is already on par, if not superior, to Darkseid in power and ability.
11) Spawn defeated URIZEN....that should pretty much end it there.
Spawn has so much on Ares. I am not saying that Ares cannot win, he may have a good chance against 3rd Age Spawn for these reasons:
1) 3rd Age Spawn retains a human body underneath his powerful exterior, meaning he is now more vulnerable to physical harm.
2) Ares is the extreme in terms of battle. He had control over all of Earth's warheads and weaponry, however.....it was said that he got knocked around by Darksied so.....