OK, some personal observations. I owned a late 2009 27-inch iMac and upgraded RAM and the hard drive to a 1TB SSD. It became amazingly fast, although current models are probably 50% faster (except for hard drive speed).
I reviewed the iMac with Retina 5K display, and the screen is just plain awesome. There's nothing like it out there. 5K displays, without the computer, are really expensive ($1,299 for an HP, $2,499 for a Dell). The 5K iMac starts at $1,999. The original $2,499 model is now $2,299 after a price reduction this year. The latter's CPU is slightly faster, but the biggest advantage is the 1TB Fusion Drive. This is a regular 1TB hard drive with a 128GB SSD that speeds up day-to-day performance to nearly the level of a dedicated SSD. It's worth the extra $299.
Remember, too, that a 15-inch MacBook Pro with Retina display is $1,999, but offers the advantage of portability.
Still, if you want the best bang for the buck, it's still a 5K iMac.