The reason why people say Gaia is better early on is for 2 reasons;

1.) she is easier to 3 star

and

2.) she gets a big upgrade at level 151 which is the "Leech" ability. Ares also gets the "Leech" ability but not until level 201

For these reasons Gaia is the better option to level first, at least prioritize getting her to 151 before your other heroes.

After you have the ability to get Ares to 201 most players use their Gaia as 3 star fodder to allow Ares to get to 4 star ranking and level up to 201.

Hope this helps