If you are looking for screen space, 15 inch is best. Trust me them 2 inches make all the difference (that's what she said)
But I am sure 13 inches will be fine. As for RAM, I use 4GB and it runs fine. RAM comes in handy when you are exporting/rendering files in DAW's, but as from the description you have provided above, 4GB RAM will be fine with 2.3GHz CPU. So 2nd option will be your best bet. However, if you like to do anything else for example 3D doing the kind of shit that Pixar does... You'll need some RAM
I hope my info was helpful.
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