Can we expect much of a difference?
Yup.
Let me get this out of the way, though..... the Xperia Z is
not a quantum leap forward from the S2. That big 5" 1080p display is nice, it's positively sexy in the hand, but performance-wise unless you obsess over benchmark tests you'd notice little difference in day-to-day use. I expect the same will be true of the S4 - until the next generation of HD games arrives that will make full use of multi-core processors and the heavyweight GPUs in the latest 'superphones'.
What we geeks (in the nicest way!) forget is that hardware specs don't actually sell handsets, features do. Most people just want to do cool stuff with their fancy smartphones, and things like S Translate, S Health and Dual Camera allow that. Or in the case of the Z, drop it in a sink and laugh at onlookers' reaction.
I
will say that the Qualcomm is very impressive in the economy stakes, and when screen-off* barely touches the battery. None of Sony's fancy 'Stamina Mode' here either.... like the S2 I'm running it with wifi permanently enabled and my social network feeds refreshing hourly. The 600-series SOC in the S4 promises even better economy, which should be matched (at least) by the new Exynos 'big.LITTLE' SOC.
* A few "Top Gear" re-runs on BBC iPlayer see the battery gauge drop faster than Ironass after a couple of shandies, though.