I can't find a geometry chart on the Performance bike. I was going to look at how they compare. (The Giant geometry is on their website) It doesn't matter how great the components are, if it doesn't fit, you won't want to ride it. The one thing that you don't want to tweak if you don't have to is fit.
A few other things to consider: I assume you are getting the Giant from a dealer? Are they throwing in tuneups or anything like that? You may want to consider that in the overall cost. Personally, I wouldn't buy a bike I hadn't ridden, unless I was sure it would fit. It also appears that both bikes have Sora components, and although I haven't ridden them personally, there are a lot of people on here who don't like them. Perhaps they will chime in with a component comparison for you. We've also heard a few horror stories on here about Performance's customer service with the bikes. You may want to check that out.
If you don't like the bike, don't get it. Look around some more, ride some more bikes and find one you love. If you love the Giant and can swing it, go for it. I've settled for less when I thought it would be better to save a few bucks, and in the long run it's better to save, get what you want, and love your bike. YOU are the one that has to ride it.