Both computers will easily handle Xcode and PS. I think the iMac may be better for one reason, the large display. In photoshop you can spread out your work, and/or zoom in. Likewise with the Xcode stuff.
With that said, you'll have to do your work at a desk, if you want to lay back on the couch while coding, go to starbucks then maybe the MBP is the solution.
The 1 TB drive is a combination flash storage and spinning disk. The disk is a 5400 rpm drive and the flash storage is faster then a typical SSD. The OS determine what files will occupy the flash portion as it monitors your usage patterns, putting often used files on the flash drive.
I have the 2TB flash drive for one reason and one reason alone. The flash portion of the 1TB drive is only 24GB (last years 1TB Fusion drive had 128GB of flash). The 2TB flash drive has 128GB of flash. I think the 24GB maybe a little too meager so I went with a larger drive.
As for performance, you will get near SSD speeds but not constant SSD speeds for all of your files. This may or may not be an issue. I think many people are very happy with the Fusion drive, I am. Its a personal decision, and I can see the justification for people wanting the SSD over the Fusion drive. I opted for a larger drive to store all my data (the 512GB SSD was too small for me).