Now that I've replaced a 2009 MacBookPro5,5 and a 2008 MacBook5,1 with a 2015 MacBookPro12,1 Retina and a 2017 MacBookAir7,2 respectively (family members), I've backed down both of these older 2009 and 2008 machines from Catalina to High Sierra.
The 2008 MacBook5,1 on Catalina was sluggish, and it's still sluggish on High Sierra, but better. Also, on Catalina there was some Chrome weirdness, with inconsistent launching. I tried Mojave but after updating the OS (even after repatching), I'd get kernel panics. No kernel panics on High Sierra, and I like the non-verbose boot. However, I can't install the last two security updates for some reason.
The 2009 MacBookPro5,5 on Catalina was also sluggish, but it's better on High Sierra, and actually is noticeably better than the 2008 MacBook5,1. I didn't think the 13% speed difference would be noticeable, but it is. Interestingly, I had no problems installing the security updates on this machine.
I don't think SSD differences can account for the difference in perceived speed. Both machines have full SATA II speeds with SSD. The MBP5,5 has a DRAM-endowed but old Samsung 840 EVO, and the MB5,1 has a DRAM-less but more recent Crucial BX100. Previously the MB5,1 had an ancient Intel 330 (SandForce) but that would only negotiate at SATA I speeds.
I'll definitely be keeping the MBP5,5:
I think I may try to re-home the MB5,1 if any local friend can actually make use of it, but I'm not optimistic since everyone has more modern machines already. I bought it about 3-4 years ago or so for $180 CAD, or about $140 US (at today's conversion rate). It was basically non-functional with 2 GB RAM, hard drive, and sticky trackpad. However, with additional RAM, cheap SSD, and a screwdriver to adjust the trackpad click, it was all working great in short order. It's served me well, and it's had a good run, but it's time for re-homing or retirement.
Meanwhile, my iMac11,3 with High Sierra feels like a modern machine in terms of speed. Too bad the OS is stuck at High Sierra. (I'm not willing to go for the super-hacked Big Sur installs.)
My youngest will likely be needing a machine in a couple of years, maybe 2023. If my extra desktop Macs - iMac11,3 and Macmini7,1 - are not suitable, then I'll look for say a used MacBookAir9,1 i3 or a refurb MacBookAir10,1 M1, or perhaps a refurb MacBookAir11,1 M2.