Well with the 12 or so hours I have had the phone I would say that Google Now is fantastic. You cannot (at this point) add this to your S3 but alot of the Touchwiz apps have similar or equivalents in the Play Store, those I want I can add without having all the other crap which goes with it.
How long will you keep your S3, 12 months and change up for the S4. How long do you wait for Jellybean 2 months, maybe 3 - thats between 1/6th and upto 1/4 of the total time you own the handset it will not be on the latest OS with the improvements that will bring.