Hippos come out better than gargoyles but upgraded gargoyles are better. here is why.
gargs come out with 3 armor, hippos with none. they both are unarmored. that being said, they both do 100% damage to each other. gargs have 410 hp, hippo have 525. hippo has a higher dps than gargs.
gargs have dps of 46.7dmg/sec, 50.9 dmg/sec on average. however gargs 3 armor gives a 15% reduction. so it is 43.3 damage hippos do. yes hippos have higher life and move at 320 speed while gargs move at 300 so it essentialy means it coems down to two units fairly evenly matched and micro wins this.
Now on to upgrades, gargs with upgrades have 9 armor, hippo have 6.
hippos have a dps of 63.8 which is dropped cause of the armor by 22, making it 41 damage. gargs have a dps of 58.5 dmg/sec which is dropped cause of the armor by 15.2 giving gargs and average dps of 43.3. hippo still have 525 hp and gargs only 410 right? well upgraded gargs can turn to stone healing 8hp/sec and gaining 15/21 armor depending on upgrades. micro rules this battle, but these numbers are pretty good. You should also rememebr gargs have a ground attack and hippos do not. hippos are also 3 food and can have an archer mount them.