I have the same Mini with 8G and a Samsung 840 evo 256GB, Mac OS El Capitan 10.11.6.
IMHO it depends on your tipical usage of the mini.
Probably the biggest impact is the SSD: the boot will be really quick and not only, consider also that SWAP on SSD is better than SWAP on spinning drive.
But also the memory can have impact, I used it as my primary PC till 2018 (I bought a 2018 mac mini) and it was OK for my usage.
Obviously now my new mini has a i7 8th gen, 32 GB of RAM and 512 G of SSD and the difference between the two minis is... really huge, but I still use the 2009 as backup or just with my TV (now it is placed just under the TV and connected via HDMI) to watch some movie (obviously no 4K, just FullHD) and after more than 10 years since I bought it I'm really satisfied and impressed about the performances (considering its age)