To make them the same price you could either upgrade to 3.4 i7 or get the Fusion Drive. Anyways the i5 is 3.2 GHz on the iMac where the i7 on the rMBP is 2.4 GHz, and there's barely a speed difference between i5 and i7s (there's one extra thing you can do to an i7, its either hyper threading or overclocking or something of the nature, I'm nor 100% sure right now) so still I'm pretty sure the i5 in the $1999 iMac is more powerful in the i7 in the rMBP.
That being said, if I were to get the iMac, I would get the i7 and a 1TB Fusion drive - if you're spendin $1999 you're already trying to get the top of the line, might as well throw those on for long term endurance and resale value.
Yeah you got a point I'm just confused on which I should get. How bad is the iMacs screen compared to the retina Mac?