There's obviously different ways to think about it. For me, I went with the base model. Put the $300 into a savings account for your next computer; you've already saved 1/4 the price of a brand new computer.
Yes the CPU is faster on the more expensive model, but you'll never notice unless you are running bench measurements or using a watch to time renders, etc.
GPU's generally don't matter unless you're a gamer. Though more applications are taking advantage of the GPU, but I'm not sure if the difference between the two GPU's is that great.
You can always buy an external drive, even one with Thunderbolt. And the internal drive is the one component that you'll probably replace during the lifetime of your computer anyway. When/if necessary, you can upgrade to a larger internal drive.