My brother has the 21 inch iMac, he's had it for a few months. (The one with the 512mb Geoforce).
I have the iMac 27" (top one). I notice the difference when I'm doing heavy photoshop or After Effects stuff. I second that the monitors on both are awesome.
I've used the retina macs (student's machine-I teach) and I think (in my experience) not as powerful as either iMac. It is a great machine for a laptop, you probably won't find a better laptop on the market. It is expensive though, and less powerful than the iMacs. Read up though, I think they don't allow you to upgrade the RAM in some macs meaning if you want the most you'll have to do it when you originally buy it. I would go with 8GB of RAM at the bare minimum.
The short answer is the more you pay, the longer the machine is going to last you because of the speed differences.
Might be worth holding out - check this.
http://buyersguide.macrumors.com/ They recommend when Apple does updates. I think I read somewhere they're going to upgrade the chips in the laptops to better, faster ones very soon.
If you're looking for a cheaper machine and money is an issue, you can also go with the Apple refurbished ones. I think they even come with the same amount of warranty as a new one. If you search around, there are guys on eBay that do the Extended AppleCare warranties for way cheaper. Just find a reputable one.