Upgrade old MacBook

[Nowindows]

Hi everyone, like most I've been waiting for this update. I can't decide between 13 inch or 15 inch.

My use for this would be email, internet, Netflix, handbrake .avi to .mp4, maybe some photos/videos editing like iMovie or something, some web designing as well.

Debating between

13" MacBook Pro Retina
2.4GHz dual-core Intel Core i5
8GB 1600MHz
256GB
$1499
Upgrade to i7 and/or 16gb memory --> $1999

And

15" MacBook Pro Retina Basic
Maybe upgrade to 16GB memory for another $200?

-What kind of difference can you expect from the dual-core vs quad-core?
-16GB "overkill"?
-Big difference between graphic cards?

Advice much appreciated!