The i7 will be faster, although with the iMac you get the 27" screen included. So you would want to factor that into your comparison. But if you can justify the extra cash and want a Mac, go with the iMac, if the savings are too great with the Dell, go with the Dell. (Also, consider that the iMac will hold its value much better when it comes time to sell it, so it make up some of the difference in value at that time.)