Lubuntu is the lightest (least memory, lowest CPU load) of the supported flavors of Ubuntu. It chooses the best of low-power, low-memory packages, yet you can add packages found in Ubuntu if you need them. I have used it in many older PCs and find it very useful.
The LTS version 18.04.3 (64-bit) (32-bit) is supported until April 2023 and uses the well-known LXDE desktop environment. LTS versions are stabler and, in theory, less problematic.
The latest version, 19.04 (download), is an interim release, and only supported for nine (9) months after its release, January 2020. That doesn't mean it will stop working, but its expected users will upgrade to version 19.10 when it becomes available, then version 20.04 (which will be another LTS). However, there's two things special about 19.04 in this context.
- It uses the LxQt 'desktop' (desktop environment), which many
folks find even faster and lighter than LXDE. It will still run the same programs ('packages') as LXDE Lubuntu and other Ubuntus, but the LxQt folks also made other changes, not replicating the LXDE look-and-feel exactly. That's why guiverc said above in comments "The change of desktop also complicates things.."
- Like all other Ubuntus now, it is packaged as 64-bit only. Your PC is 64-bit, so it will run, but some folks like 32-bit versions on older PCs because they consume less memory. My personal suggestion is to try 64-bit (either 18.04.3 or 19.04) and see if you need to change to 32-bit to pull every last bit of performance out of your system.
Then, there's Xubuntu, which uses an older desktop, xfce. There is an LTS version (64-bit) (32-bit) with support until April 2023, and an interim 19.04 version (64-bit only) with support until January 2020, the same as all Ubuntus. It is not as lightweight as either Lubuntu with LXDE or LxQt, but as with Lubuntu, you can add other Ubuntu family programs. guiverc mentioned it in that comment above.
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