SSD vs HDD while editing

but don't be fooled into thinking just because there are no moving parts (beyond the molecular level I suppose) that they'll last forever.  They apparently have a limited number of read/writes.  Although I've been using them for a number of years for startup drives on my decrepit macpro and macbookpro and haven't seen any outright failures, I have seen corruptions that required restoring from a clone.  

Whenever possible, use SSD since it is approximately 5 times faster than an HDD for both writing and reading, so if you work with very large video files it will be easier for Premiere Pro to process the data back and forth.

tell us what kind of work you're doing.  HD, 4k, a lot of effects, multicamera and if so how many angles all will be helped by using ssd's for your media.  If you're working with relatively simple projects it may not make that much difference as far as your media drives go.  Booting from a ssd has made a lot of difference for me.  And yes, working with proxies lessens the need for ssds for your media drives.   I mainly work off of HD's for media without any issues.  I'm still primarily working with HD material and even when I'm working with 4k sources, I'm usually finishing as HD.  For me, the price of HD storage vs SSD storage is important.  I'm often working on complex documentary projects that have a lot of media but not very complex fx, layering, etc.  

Nowadays the HDD is only for backup, it's not really suited for editing. We only used them cause that's what was available. There's a big difference to having it on your SSD. Better if you have two. PP on your boot SSD, then all your footage and projects on the 2nd SSD. It will be a big difference.