Fusion Drive vs SSD internal HD

Hi Folks,
I'm ordering a new 27" 5k iMac this week with all the upgrades including the 4.0GHz i7 processor and ADM 4GB video memory graphics card. I'm finally jumping from my 2010 27" iMac running FCP 7 to this new machine running FCX.
My question revolves around the internal HD choices: 3TB Fusion Drive or 512 SSD?
Money is not the real issue as there is only a $100 difference between them.
I work with an external, 4 disk, USB 3.0 connected array where my video media lives (not the internal HD) If I got the fusion drive, I'd create a second, 2TB partition for storage on the drive, thus keeping the 128 Fusion SSD and 1TB of HD space available to the OS.
This would be convenient, but HOW MUCH slower might this be instead of going with the 512 SSD instead? If I went 512 SSD, I'd have to install an external HD for storage which is less convenient due to noise, cost, extra power supply etc.
I've done a lot of reading on this, but would appreciate any real world opinions you might have.
Thanks in advance,
Chris McLean
Beyond Film

I have six Macs, four with SSD and two with FD. My main editing computers are a 2015 iMac 27 with 1TB SSD and a 2013 iMac 27 with 3TB Fusion Drive. All my media is on a Pegasus Thunderbolt RAID array.

While SSD is much faster in I/O benchmarks, in general there is not a huge difference in FCP X editing performance if your media is external. In fact the editor of the Will Smith movie "Focus" used an iMac 27 with 3TB Fusion Drive, with media on a 32TB Thunderbolt RAID array.

On larger multicam HD productions, or if acquisition is in ProRes, raw or 4k, the media won't remotely fit on a 1TB SSD so you must use external storage anyway. If you must use external there's a good argument for SSD. It can be consistently faster for loading apps, rebooting, updates, etc -- vs a Fusion Drive with a fluctuating cache state. The extra storage space of FD encourages use of that, yet you generally don't want media on the boot drive. As FD fills up it will gradually slow down since the 128GB SSD has more trouble retaining frequently used data. However unlike a pure HDD, Fusion Drive performance does not fall off a cliff as it approaches 90-95% full.

I personally would not partition a FD to be separate 128GB SSD and HDD partitions. It works pretty well in full automatic mode. The big benefit is the self-managing nature and it's a lot faster than a plain HDD.

Re noise, space, etc of an external HDD, the 4TB Seagate Backup Plus Fast is a USB 3 bus-powered drive that is very small and requires no external power. It is pretty fast -- about 200 to 250 MB/sec. It is internally RAID 0 so you'd need to ensure good backups.

I am satisfied with FCP X performance on my iMac with 3TB Fusion Drive, however my new iMac has SSD.

Thank you for your thoughtful and thorough explanation. Your experience was very helpful to me.
Chris