The i3 is slightly generally faster than the 6300 (Check the GTX 760 benchmarks in the link below), but the 6300 can easily be overclocked to catch up. (Will even exceed the i3 when overclocked a good amount)
Both processors will do the job just fine, But if you are playing anything poorly optimized that requires strong singlecore perf like GW2 take you may want to consider running the i3 instead of the 6300.
Running AMD systems won't really be much of a problem if your case has good airflow (The CM N200 is perfect for that, Has 2 fans that are plenty for the whole system) unless you are in extremely hot climate. Back down overclocks during summer and clean your PC of dust every 2 weeks or every month and it should be fine.
GTX 960's may or may not include witcher 3 with the purchase, contact your dealer for details. (If it has Witcher 3 for free advertised on the box it will always contain a code, though)
http://us.hardware.info/reviews/5766/3/amd-fx-6300-vs-intel-core-i3-4330-budget-gaming-cpu-deathmatch-performance-in-games