Will upgrade to Android 4.3 with Samsung Knox make my s3 impossible to root? So my Galaxy s3 has finally offered to upgrade to 4.3.

I've heard Android 4.3 contains Samsung Knox which I have heard is a security feature that will make it impossible or at least really risky / difficult to root.

Is this true? Will I still be able to root my phone after I upgrade to Knox?

If I plan to root my phone after my warranty expires, should I not take this upgrade?
Depending on what version of S3 you have (there are 13 just looking at the wikipedia article), there could be different safeguards in place to prevent tamporing
Having KNOX installed is one thing, upgrading to 4.3 is another as some of the updates that Samsung has put in prevent certain types of expolots to get the bootloader unlocked and ability to boot a unapproved rom
A coworker of mine upgraded his phone to 4.3 and serious issues with KNOX not letting him boot. He had to downgrade all the way back to 4.1 and then upgrade back up to get a working phone. Needless to say, all of his data is lost.
If you are planning on rooting, do it now, upgrade to a rooted Rom with TouchWiz (like Carbon), or go full 1337 and get a CynogenMod rom and run Kit Kat
Rooting your phone kills the warranty, but there are ways to revert back to stock. Depending on the insurance you have on the phone, they'll take in a broken rooted phone. However, if some phone component breaks, and you have no way to get it back stock, you are sort of SOL. 
There is a huge market for used phones, and if you have yours rooted, you give a bit of added value to the phone. Swappa is probably the best place to go out and sell your phone.


TL;DR: Root now, install a rom you like, get KitKat if need be, sell on swappa.

Source: Rootin' since 2009. Owner of multiple Samsung phones.