All developers always use 100% of the hardware with every game: After all, why would you not? The improvements are with the software dev kits themselves, and I believe that Sony are ahead of MS in this area right now. Combine that with the ESRAM of the Xbone being more awkward to use initially (i.e. when programmers are not used to programming for a brand-new console), and you have a perfectly sensible reason as to why the third-party games are different so far.
Look at Forza, and look at Drive Club, both 1st party games (or at least, published 1st party). Forza looks better to me, and is ready for launch now at 1080p and 60fps. Drive Club isn't ready, but will hit the same performance targets. That says to me that once programmers get used to the Xbone, you'll notice precious difference until we are much, much further down the line. I'm talking 5+ years, when the hardware will be relatively ancient but the dev software will be pushing the limits. That's when the extra grunt of the PS4 GDDR5 may well be shown, but that's ages away.
Tbh, if all you want to do is play CoD/FIFA/AC then you might as well get a PS4. It's cheaper, after all. I want Kinect, and I want Forza, and I want Titanfall, and I want Halo, and I want XBL. It's the exclusives that should determine the system you buy, not whether one game has more pixels than another. When you buy a TV, do you pick the one with the best specs, or do you pick the one that actually looks better to your eyes and suits your needs?