i7, just because. Also, LR and other software are almost completely CPU dependent so every bit of raw CPU power helps. Getting a third party (e.g., Crucial, OWC) upgrade to 16GB should be cheaper, though I don't know how much cheaper.
Also, having more GPU memory should in theory help with driving extra monitor/more resolution. Whether or not you'll see the difference in your computer usage is a different matter entirely.
Wait, you're not getting an iMac 5K?