This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
There are soem differences between JDK 5 and 6 (and SCJP 5 and 6), but I wouldn't say there are really major differences. Obviously, code that was written using things that only exist in the Java 6 API will not compile on JDK 5.
I think that when you've prepared using JDK 5, you'll already know at least 90% of what you need to know for SCJP 6.