My personal opinion (not the person you asked but I'll provide it), a 5400 RPM drive has no place as an OS drive anymore. I was using a 2010 MBP with a 5400 RPM drive at work and it was AWFUL, my 2012 Mini came with a 5400 RPM spinner and it was a LITTLE better but there's just no comparison between that and an SSD. Now, having a spinning drive as a scratch disk or temporary holding location for stuff may not be too bad of an idea (that's what the spinner in my Mini is used for).
If I had to choose between getting an SSD or getting another 8 GB of RAM, I'd probably choose the SSD.
I find if manageable. Considered putting in a SSD many times. Considering the risk of breaking something, the trim issue which now seems solved though, the relatively expensive SSD I have not put in one yet. Sure you need more time to boot. But I code on it, multitask, listen to music, make screencasts etc. When you are about to buy a device the fusion seems nice though.
Because they are both SSD based these two storage systems have similar performance. However, in spite of the fact that the difference is barely perceptible, the SSD is faster.
The advantage that a Fusion Drive has over an SSD is cost per gigabyte. Unless SSD prices decrease dramatically the magnitude of this $ advantage will remain comparable to the speed advantage that SSDs and FDs hold over HDDs.