SSD is most likely the cheaper upgrade, and will be much more impactful. 4gb to more will be less immediate feeling unless you’re constantly running out of ram.
My laptop had 4GB RAM and a hard disk. I upgraded my RAM to 12GB ( yep! single channel, but for me 12GB single channel is better than 8GB dual channel ) and added a 512GB SSD.
If you are okay with taking some time to launch applications but your applications need a lot of memory then upgrade your RAM.
If you want your system to boot quickly and open all apps pretty fast use an SSD.
Do both if you can afford it
If you still feel slow switch to XFCE
One thing I regret switching to an ssd is my Plasma splash screen turns off in a second
I just changed my SSD to a HDD to test it, and the system needs three times longer to startup and to launch a program. So, if I had to decide between RAM or SSD, I would choose the SSD first. After that I won’t buy me a coffee until there’s enough money for more RAM.
Tested on a HP G7 with 4/8 GB RAM and SSD/HDD.
The only reason to upgrade your RAM before upgrading to SSD is using Software like Virtualbox. In that case you need definitely more Memory.