The Yeerks were after our bodies- I believe silent invasion was the right choice for them, simply because humans would die by the billions rather than become enslaved. Sure, in my opinion they would have stomped us in all-out war, but how many hosts would they have lost? We're nearly as valuable to them as we are to us. The main difference is that they're more than willing to sacrifice the Earth to get at us. Oh, and, like Terenia's saying, if they do lose here, they'd have no problem just wiping us out. War with the Yeerks is interesting because they're after more than our destruction- they have to take as many of us as possible alive.
Yeah, the silent invasion definitely favored the Animorphs. I can't imagine them having been all that important to the war if it took place in the open, and a fair amount of the Yeerk resources were spent trying to cover up what the Animorphs had done. Good thing they were only invading one one front, eh? A second Yeerk Pool in New Hampshire or wherever would have kicked Earth's butt.