An Apple Fusion Drive is the fusion of two physical drives (HD & SSD) into one logical drive. Apple uses an algorithm within the OS to locate more used files onto the SSD and less used files on the HD.
Basically a Seagate SSHD is very similar to an Apple Fusion drive setup. The biggest difference is the size of the SSD cache on the Seagate drive is smaller and the firmware within the drive handles the more accessed files on the deep cache SSD.
Both offer improved performance over a standard HD. We use both depending on the age of the system.
It sounds like you are about to buy a Mac Mini and are trying to figure out which way to go. The newest model of the Mini does not offer a dual SATA drive config, instead the new Mini has a single SATA port and a M.2 Blade SSD port. Depending on the model you buy you may find your options limited. So far Apple does not offer the custom SSD cable & Blade SSD so if you are thinking on upgrading later it may not be possible,
I would recommend you get the SSD or the Fusion Drive model.