SSD is dynamically faster than the traditional HD and one must switch to SSD. First understand why, We all have RAM and what RAM do is it store the core functions or resources in it for faster processing like caching do. SSD will add more speed to it. You can locate your OS in your SSD so it will boost faster and applications will work more faster than the traditional approach. Additionally you can keep your professional files in SSD like if you are a painter then you can keep Adobe abd other design software in SSD. If you are a developer then you can keep Android Studio or other development IDE in your SSD. SSD is limited in space so you have to select the things wisely.
SSD is better for long loading things like OS, video editing apps, some games with long loading (like Sniper Ghost Warriors 3, GTA V, etc).
If you use SSD for OS, because the read + write speed is quite fast, you may have a problem to enter the BIOS. Usually we have a few seconds to enter BIOS with HDD. But if you use SSD, the screen which prompt us to enter the BIOS will be very fast, so if your SSD is the top model(PCIe SSD), there's a high chance BIOS screen will be skipped. This is one important thing you need to think before you install an OS to SSD. Oh yeah, don't use SSD for torrent or as a cache disk for download manager, that will reduce SSD lifespan quite fast.
I personally have an SSD disk in my laptop and let me tell you that recently I have been using it as a primary disk to install and run programs from and the change in speed, processing rate and use of memory is remarkable. What I recommend you to do is to install in there the OS and the heaviest programs you use more because as the speed of processing and loading these disks is faster, your computer will need less time and resources such as RAM to open programs or simply start the OS, while if you have a conventional disk use that to store documents and other information.
My laptop for example is from 4 years ago and recently a friend who has a macbook was astonished because mine starts session faster than the one that he has.