Save money on iMac memory too: OWC memory for iMac.
The new iMac 5K and iMac 4K are available for pre-order at B&H Photo*. This site gets credit if you order through links on this site.
MPG recommends at least a 512GB SSD (flash) drive regardless of which model is chosen. For some, the Fusion drive option can be a good choice. Avoid hard-drive only models, as this is molasses-slow—and iMacs are a headache to upgrade the internal drive.
But here’s the trick: get the iMac 5K 27" with 8GB memory + M395X graphics, saving $3o0 to $400 by getting a 32GB or 64GB memory upgrade from OWC instead of buying 32GB directly (memory is super easy to install). And since Apple won’t sell you an iMac with 64GB, that’s the only option those wanting 64GB (without wasting money).
The 21.5" iMac 4K can be a good choice for casual and routine uses (think of it as buying a nice display), but stick with 512GB flash and/or 2TB Fusion drive models for best performance. Note also that the 21.5" iMac 4K has (yuck) soldered-on memory limited to 16GB, so consider nothing less for memory.