The processor speed...I doubt you'll notice any difference. What you're really buying there is the better video card and 50 extra GB of hard drive space. Is the video card worth it...for some, yeah. But for you?
I'd suggest getting the cheaper model and giving it 4 GB of RAM ($200) and buying 2 USSB hard drives for it. (Western Digital MyBook 500 GB is $100.) Get a little USB hub too, so you'll only use 1 port on the computer. Use 1 drive for file storage and the other drive for time machine to back up the laptop and the file drive. (Remember to format each drive as Mac Extended when you get it using the Disk Utility.)
Now you've spent $400 instead of $500 and you'll have more RAM, more storage, and a good backup solution.