Which image should I pick?
If you are wondering which download to pick, here are a few pointers on the options.
Ubuntu 18.04.4 LTS vs Ubuntu 19.10
Generally we recommend the Long Term Support (LTS) releases as they are supported for five years of free security and maintenance updates, guaranteed. However, the interim releases often have the newest features and hardware support. Ubuntu 19.10 comes with nine months of support, until July 2020, so there is really no bad choice.
32 bit vs 64 bit
The Raspberry Pi 2 only supports 32 bits, so that’s an easy choice. However the Raspberry Pi 3 and 4 are 64 bit boards. According to the Raspberry Pi foundation, there are limited benefits to using the 64 bit version for the Pi 3 due to the fact that it only supports 1GB of memory; however, with the Pi 4, the 64 bit version should be faster.