Hello and welcome to the forums
1_There is no performance difference, but 4gbx3 is "usually" better for OC'ng and requires less slots
2_No, there are many RAMs that aren't in the QVL list but are supported.
Where did you read about that ? (the required dimms)
3_The second one has lower timings and only $5 cheaper,so go for it
As for my suggestion,well i think even 6GB is more than enough for today's games/apps so you may want to consider it too,what's your usage ?