For most people a fusion drive will perform the same as an SSD - the computer will optimize your recent/most used files and applications to be on the SSD side of the drive. A pure SSD does have some performance benefits with pro/creative applications that write a ton of data/temp files, but based on what you're saying, routine computing, xrays, etc - the fusion drive should perform about identical 99% of the time, but with the additional storage capacity. After the 128gb SSD fills up, it will start storing older files on the HDD side, and bring them back as needed.
Note that the 1TB fusion iMac has very limited SSD storage, something like 24gb - I'd recommend getting the 2TB which has 128gb on the SSD.
There have been rumors of a refresh for a while - it may happen within the next few weeks - Apple is expected to have an event late march or early April. If they don't announce it then, it will likely be several more months minimum. So not the best time to buy, something will come probably sooner than later.
Last thing to note - about 16gb ram - Apple charges crazy prices. On the 27" iMac you can upgrade ram yourself through a port in the back, it's plug and play. Check out OWC or Amazon for 3rd party ram, you can order the same stuff much much cheaper. If they announce a new iMac in a few weeks the exact ram specs could change so be aware of that.