This is really quite easy - get yourself the Google Nexus 4. I reviewed the Nexus 4 here and called it the best smartphone in the world right now, as (in my opinion) it really is. It's fast, it's gorgeous, it's feature-packed, and future-proof.
Google's Nexus-branded devices are always the first to receive the latest iterations of the Android OS - another huge benefit of owning the Nexus 4. I understand you'd like to continue owning a Sony device, continuing your legacy of owning Sony smartphones - but as you said, it's only a dual-core smartphone, where the Nexus 4 sports a quad-core processor and 2GB of RAM.
Yes, they're both the same price, but the Nexus 4 is heavily subsidized in terms of its price - it's like a $800 smartphone, for just $349.