I would say they are equal in strength (and the spino is also in the same tier), and should be used depending on the situation.

Giga has slightly more base health (1150 vs 1100 when tamed), but slightly less base damage (70 vs 75 when tamed) than the rex. The giga bite causes a bleed that does a percentage DOT damage, while the rex bite are better at penetrating armor. Rex can quickly regenerate health by eating carcasses and grab small dinos, while giga is faster and can deal up to 50% more damage when enraged.

We tested rex vs giga in pvp. With 80 armor saddles, same health and damage, giga defeats rex by bleeding, which deals massive damage. However, the Tek rex defeats the Tek giga under the same conditions, because Tek dinos are immune to bleeding and the giga deals less raw damage against a target with a good saddle.

irl i mean