As you are going to use the iMac for Final Cut Pro X and Aperture, you should get the best graphic card and processor. The processor that comes with the 27-inch iMac is powerful enough to run both without any problem, and respecting to the graphic card, the default graphic card will work properly, but you have a NVIDIA GeForce GTX 680MX that will provide a better experience.
If you don't have enough money for those upgrades, the basic 27-inch iMac will run properly
Aperture is and will remain a resource hog demanding and appreciative of any and all resources you can make available to it. IME, the processor speed is the ultimate performance limiter. Get at least 8 GB of RAM, get an SSD for your system drive, and get the fastest processor that is reasonably priced.
A more specific answer requires knowing your use of Aperture, and your tolerance for delays at different stages of your workflow. For me, I don't see much difference between Libraries on an SSD vs. on a USB-3 drive (and lately I've been dealing with very large (10,000 x 10,000 px.) files). Batch changes to thousands of files always takes time. Editing is usually instantaneous. If I could magically double _something_, it would be processor speed.
300 GB is large. Others will, I hope, have something to add re: FCPX.