I don't know about you, but I've been using both... Ifrit for trash mobs, since I can normally put him on Sic and he does more dps than garuda that way, unless we need to sleep adds, in which case I put him on obey, until last target, and then change it to sic...
During trash mob fights, it's normally not worth using contagion, as mobs die too fast and then i find really little use for garuda, I really think Ifrit is so much better to deal with trash mobs, so much faster...

I do find use in ruin 2, it is a good finisher, when a mob is near dead, I usually combo ruin I > ruin II, for a fast 160 potency to bring down the mob and move to the other one. During trash mobs you really can't run out of mana, seriouslly...

Now, when It comes to boss fights, Garuda is infinitly better, using Contagion to extend your raging strikes dot's, plus thunder/miasma 2 just pops your damage output skyhigh... And it makes so much easier to manage the dots over the boss. It makes Fester management a lot easier.

Overall Garuda is better, but during trash mob fights, there is just no time to enjoy the gains from her Contagion, mobs come down just too fast.